Rho adsorbent compositions, methods of making and using them

ABSTRACT

Disclosed herein are novel RHO zeolites useful as kinetically selective adsorbents for oxygen and/or nitrogen. The adsorbents can be used in pressure swing adsorption processes for selectively adsorbing oxygen and/or nitrogen from feed streams such as an air stream or crude argon stream. Also disclosed are novel methods of preparing RHO zeolites, including in particular mixed-cation RHO zeolites.

TECHNICAL FIELD

This disclosure relates to adsorbents, methods of making the same, andprocesses that use them. The adsorbents are zeolites of the RHO-type(hereinafter referred to as RHO zeolites). The adsorbents disclosedherein are useful for various applications, such as for kineticallyseparating oxygen from oxygen containing streams such as for purifyingcrude argon, for separating nitrogen from air, for removal of trace N₂from argon, and/or for removal of CO₂ from methane. In particular,disclosed herein are RHO zeolite adsorbents that are kineticallyselective adsorbents for oxygen, effective at ambient or sub-ambienttemperatures.

BACKGROUND

In the gas production industry, there is a need to efficiently separateoxygen from oxygen containing streams at ambient or sub-ambienttemperatures.

In cryogenic air separation, nitrogen (N₂), oxygen (O₂) and argon (Ar)are separated based on their boiling points and relative volatilities. Afirst cryogenic column provides a rough separation of the three maincomponents of air: N₂ (78%), O₂ (21%), and Ar (1%). A side stream isremoved and sent to a second column known as the side arm column orcrude argon column. This stream is called “crude” because it exits thisside arm column at only about 95% argon. The conventional methods offurther purifying this crude argon are limited to: “Deoxo” purification,getter bed technologies, and additional distillation. The Deoxo processreacts controlled amounts of hydrogen with the oxygen in the argonstream to remove the oxygen. Because the reaction of hydrogen and oxygengenerates significant heat, this process can be dangerous if notcontrolled properly. Getter beds only function at lower oxygenconcentrations by reacting oxygen with copper catalyst to form copperoxide. When high purity argon is desired, a third distillation columncan be used to further concentrate it. Unfortunately, these distillationcolumns require upwards of 200 stages due to the similarity in boilingpoints of O₂ and Ar and are less economical than is desired.

To achieve a kinetic separation of O₂ from either N₂ or Ar by anadsorption mechanism, an adsorbent structure must be developed with veryspecific pore dimensions. The Lennard-Jones 6-12 kinetic diameter of Ar(3.40 Å) is smaller than that of O₂ (3.46 Å), but O₂ is not a sphericalmolecule and has a minimum molecular dimension that could be exploited.The symbol A represents the Angstrom, a unit of length, which is definedas 10⁻¹⁰ meters. Adsorption mechanisms suggest that the minimummolecular dimension is the limiting factor for kinetic exclusion. Withthe proper orientation, O₂ should diffuse into a pore with an effectivediameter of 2.8 Å. Argon, a spherical atom, will have a constantdiameter of 3.4 Å. This 0.6 Å difference in diameters is the keysensitivity that an O₂ selective adsorbent must demonstrate to achieve akinetic separation between oxygen and argon. With such an adsorbent, aprocess could be derived that purifies crude argon from the cryogenicair separation process in a safer and more economical manner and removesO₂ from argon much more rapidly and efficiently.

Compared to the conventional, very elaborate methods of recovering argonfrom a crude argon stream, a PSA (pressure swing adsorption) processprovides a simple and effective alternative for argon purification andrecovery. No hydrogen or additional cryogenic stages are required.However, removing oxygen and nitrogen simultaneously from crude argonstream requires either two separate PSA stages or a PSA column (PSA bed)comprising two layers of adsorbents with different utilitycharacteristics.

Kumar et al. in U.S. Pat. No. 4,477,265, discloses a two stage VSA(vacuum swing adsorption) process for argon purification. The two layersof adsorbents for oxygen and nitrogen removal are in two separatedstages. The two stages are connected in series. This allows the processto be more flexible. For example, it permits possible bed interactionseven within a stage and using different number of beds in differentstages. In one preferred embodiment, three beds are in fact used in thefirst stage for nitrogen removal using a nitrogen equilibrium selectiveadsorbent. Two beds are in the second stage for oxygen removal using anoxygen rate selective adsorbent. The basic cycle steps includeadsorption, evacuation, and pressurization. Also, argon recovery is low,and recycling the waste stream, still containing considerable amount ofargon, back to cryogenic unit is necessary for additional recovery.Recycling of VSA waste stream back to the cryogenic plant makes the airseparation unit more complex and a VSA option less attractive. It isimportant to note that a VSA instead of a PSA process is used in thelayer bed configuration.

Pressure swing adsorption (PSA) processes comprising several layers ofadsorbents are known in the open literature. However, the arrangement ofthe two layers of adsorbents with different characteristics in the samePSA column requires careful consideration. For example, one layer may becomposed of an adsorbent with kinetic selectivity to the contaminantgas, where the product gas has very slow diffusion kinetics relative tothe contaminant gas. Another layer may be composed of an adsorbent,which separates gases based on differences in equilibrium capacities,where the contaminant gas is more adsorbed than the product gas. Theapplication WO2008072215A2 discloses such a PSA process for upgradingnatural gas. The novelty described in this application is thenon-conventional arrangement of an adsorbent with smaller capacity andperforming a kinetic separation, followed by an adsorbent performing theseparation by differences in the adsorption equilibrium of the species.Note that only one component is removed by the two layers.

U.S. Pat. No. 5,730,003 describes a hybrid process where crude argonproduced in a cryogenic distillation plant is processed in a 2-bedpressure swing adsorption (PSA) unit to produce 99.999% argon. If thecrude argon contains significant amount of nitrogen in addition tooxygen, the patent reports to include a nitrogen selective adsorbent ina layer separate from the oxygen selective layer. Carbon molecular sieve(CMS), type A zeolite, clinoptilolite, and the adsorbents disclosed inU.S. Pat. No. 5,294,418 are used as an oxygen selective layer. As anitrogen selective layer, adsorbents such as CaA, type X zeolite (LiX orNaX), and zeolite of type A & X containing mixed cations selected fromgroups I and II of the periodic table (LiNaX) are mentioned. Thelayering preference, PSA feed temperature and regeneration conditionsare not reported. In the description of the PSA process, an optionalvacuum pump is incorporated. It is not clear whether the adsorptionprocess operates under pressure swing or vacuum swing mode forsimultaneous removal of oxygen and nitrogen from argon stream using thelayered bed.

Carbon molecular sieves (CMSs) have been developed that selectivelyadsorb O₂ over N₂ based on the smaller kinetic diameter of the O₂molecule, see e.g. Yang, R. T., Gas Separation by Adsorption Processes,Butterworths, Boston, 1987. More recently, in S. U. Rege and R. T. Yang,Adsorption, 2000, Vol. 6, 15-22; and U.S. Pat. No. 7,501,009 to Graham,et al., CMSs are proposed for removal of O₂ from Ar by kineticseparation.

Nevertheless, use of CMSs for the purification of crude N₂ or Arpresents several problems, including, but not limited to, a potentialfor combustion if CMS dust is returned to the cryogenic distillationtower and low recovery due to variable pore size distribution. Zeolitesare porous aluminosilicates, which are non-combustible, more stabletowards oxygen exposure than CMSs, and contain well controlled poresizes based on their highly crystalline structure. Thus, they have thepotential to overcome many of these drawbacks.

An inherent problem with many kinetic PSA processes for the purificationof crude N₂ or Ar utilizing either zeolite or CMS adsorbent is lowrecovery of the desired Ar or N₂ product due to low utilization of thefull capacity of adsorbent. This arises where the feed step must bestopped well before adsorbent saturation to avoid contamination of theprimary product with a high level of the impurity. In such case, it isnecessary to recycle the PSA waste stream, still containing significantamount of argon or nitrogen, back to the cryogenic air separation unitfor additional recovery. An improvement in kinetic selectivity wouldenhance the bed utilization, and thus the final argon or nitrogenrecovery.

A RHO zeolite has a symmetric, three-dimensional pore structurecontaining channels with openings made up of two 8-membered oxygenrings. The nominal ring diameter or opening is 3.6 Å. This is close tothe target pore dimensions, mentioned above, for the kinetic separationof O₂ from Ar and N₂, and N₂ from Ar vide supra. This pore dimensioncould also be useful in the separation of CO₂ from methane.

RHO zeolites require the presence of large cesium extra-frameworkcations as the structure directing agent during synthesis, and do notoccur naturally. They were first prepared in 1973 by Robson andcoworkers (Advances in Chemistry Series, 1973, 121, 106.). This initialsynthesis used no additional organic templating agents and produced RHOmaterials with a ratio of Si to Al atoms equal to 3.2, hereafterspecified by the shorthand RHO(3.2). More recently, RHO zeolites havebeen synthesized by Chatelain and coworkers using 18-crown-6 as atemplating agent (Microporous Materials, 1995, 4, 231). The templatedmethod gives highly crystalline RHO with Si/Al=3.9 to 4.5, i.e.,RHO(3.9) to RHO(4.5). The preponderance of structural work with RHO hasbeen carried out with RHO(3.2) and RHO(3.9) materials. RHO compositionswith Si/Al>4.7 have been reported by Mitsubishi Chemical in WO15020014A1through a mixing modification of the Chatelain procedure. Theyspecifically claim copper and iron exchanged RHO materials at thesehigher Si/Al ratios for NO_(x) reduction applications, but mention othertransition metals, including zinc, in their background. The use of anyof these materials in air separation is not mentioned. Lower Si/Alcompositions with only copper, iron, or zinc are not taught, and it isnot obvious that they would be stable, based on the greater basicity ofthe lower Si/Al RHO materials. In U.S. Pat. No. 5,944,876, Corbinteaches of fully and partially cadmium (Cd) exchanged RHO zeolites, withSi/Al>3, including RHO compositions with at least 1 Cd²⁺ cation per unitcell, with an assortment of other cations, including Zn²⁺. Because ofthe size of the Cd²⁺ cations, these compositions require at least onecation, namely the Cd²⁺ cation, to reside in an 8-ring position. Corbindoes not teach how to prepare Cd RHO materials with a balance of Zn²⁺cations. It is not obvious that RHO compounds with 3<Si/Al<5 would bestable if <one cation per unit cell was required to occupy one of the8-ring positions, due to the greater basicity of lower Si/Al RHOcompositions.

Higher Si/Al RHO materials with Si/Al>5 have been prepared by use ofexcessive templating agent (Ke Q., Sun T., Cheng H., Chen H., Liu X.,Wei X., Wang S. Chem Asian J., 2017, 12, 1043.)

The as-prepared, hydrated, RHO zeolites crystallize with acentrosymmetric body centered cubic (bcc) structure, but it has beenshown that this structure can undergo rather large distortions to lowersymmetry upon dehydration and depending on the type of extra-frameworkcation substitution. The distortion, which can be observed as a largeunit cell contraction, is largely driven by the distortion of the RHO8-rings. Corbin and coworkers have shown that the undistorted,essentially circular rings of the proton exchanged RHO can distort tohighly elliptical rings on exchange of small, high charge densitycations such as Ca²⁺ and L⁺ (Journal of the American Chemical Society,1990, 112, 4821).

In principal, this distortion mechanism could be used to tune the ringsize, shape, or diameter to selectively adsorb certain gases over othersby size exclusion processes. This mechanism has been exploited by Cormaand coworkers (Chemical Communications, 2012, 48(2), 215) and Wright andcoworkers (Journal of the American Chemical Society, 2012, 134, 17628)to achieve large equilibrium selectivity for CO₂ adsorption overmethane. In U.S. Pat. No. 7,169,212, Corbin describes the use ofmixed-cation RHO zeolite,Li_(7.1)Na_(1.93)Cs_(0.3)Al_(11.7)Si_(36.3)O₉₆, also specified here bythe shorthand notations of Li_(7.1)Na_(1.93)Cs_(0.3)RHO(3.1) forseparation of oxygen from nitrogen in a PSA process. In this last case,though the kinetic selectivity for oxygen vs. nitrogen adsorption isextremely high, the RHO 8-ring size has been made sufficiently smallthat the uptake of even the smaller gas, oxygen, is extremely slow andis not practical for standard PSA applications.

Several researchers mention equilibrium-based separations of O₂ from Ar,but there are few rapid kinetic separations reported. Most of thesekinetic separations use carbon molecular sieves (CMS), which, bysize-selectivity, are able to adsorb O₂ at rates about 30× faster thanAr at ambient temperatures (U.S. Pat. No. 6,500,235 and S. U. Rege andR. T. Yang, Adsorption, 2000, Vol. 6, 15-22). U.S. Pat. No. 6,500,235also mentions the use of a transition metal containing metal organicframework (MOF). Both materials show relatively slow uptake and, becausethese materials contain organic components, the adsorption process wastestreams must be filtered if the streams are to be recycled back to acryogenic plant. CMS materials are typically not suitable for separationof N₂ from Ar, because they have essentially no equilibrium selectivityand limited kinetic selectivity between these two gases.

Only a few zeolite materials have been reported for the kineticseparation of O₂ from N₂ or Ar at ambient temperatures. S. Kuznicki, B.Dunn, E Eyring, and D. Hunter (Separation Science and Technology, 2009,44:7, pp 1604-1620) report the kinetic separation of O₂ from Ar usingthe Ba exchanged titanosilicate, Ba-RPZ-3. Rates appear to be slow forthis material and the kinetic selectivity of O₂/Ar is less than 10. S.Farooq (Gas Separations and Purification, Vol. 9, No. 3, pp 205-212)describes the possible use of a modified 4A material, RS-10, from UOP.This is the only commercial zeolite based material which, at ambienttemperatures, shows kinetic selectivity for O₂ adsorption over N₂ and Arcomparable to CMSs, at similar O₂ uptake rates to the CMS materials.

D. W. Breck (Zeolite Molecular Sieves, Robert E. Krieger Publishing Co.,1984) describes the use of zeolite 4A at sub-ambient temperatures forkinetic O₂/N₂ and O₂/Ar separations.

Kovak et al. in U.S. Pat. No. 5,159,816 mention the use of a list ofzeolite adsorbents including 5A, 4A, mordenite, 13X, and chabazite forremoving N₂ and O₂ from Ar in a cryogenic TSA process.

Therefore, it remains desirable to develop adsorbents that are usefulfor the separation of O₂ from mixtures containing N₂ and/or Ar with atleast comparable kinetic selectivity to the CMS materials, but capableof operating at much higher adsorption and desorption rates. It alsoremains desirable to develop adsorbents compositions that are useful forthe removal of low levels of N₂ from Ar. It further remains desirable todevelop adsorbents that are useful for removal of CO₂ from methane.

SUMMARY

Disclosed herein are novel RHO zeolite compositions that aim to addressthe above and other goals. Also disclosed herein are processes usingsaid RHO zeolite compositions, and novel methods of making RHO zeolitecompositions.

In one aspect of the present invention as disclosed herein, there isprovided a RHO zeolite having a Si/Al ratio of from 3.2 to 4.5 andcontaining non-proton extra-framework cations, wherein the zeolitecontains at most 1 proton per unit cell, and wherein the size, numberand charge of the extra-framework cations that are present in thezeolite are such that 1 or fewer non-proton extra-framework cations perunit cell are required to occupy 8-ring sites. In certain embodimentsthe zeolite has a unit cell axis length of from 14.23 Å to 14.85 Å. Inparticular, it may be preferred that the zeolite has a unit cell axislength of from 14.23 Å to 14.55 Å, or has a unit cell axis length offrom 14.45 Å to 14.85 Å.

In further aspects of the invention, processes of adsorbing oxygen froma feed stream containing oxygen, and/or of adsorbing nitrogen from afeed stream comprising nitrogen and argon, and/or of adsorbing oxygenand nitrogen form a feed stream comprising oxygen, nitrogen, and argonare provided, in which one or more RHO zeolites of the invention asdescribed above are used as adsorbents. In preferred embodiments, saidprocesses are pressure swing adsorption (PSA) processes.

In yet further aspects of invention, methods of preparing RHO zeoliteshaving desired Si/Al ratios and methods of preparing a RHO zeolitecontaining specific extra-framework cations are provided.

Accordingly, the present invention includes the following aspects(referred to as Aspects 1 to 27):

Aspect 1. A RHO zeolite having a Si/Al ratio of from 3.2 to 4.5 andcontaining non-proton extra-framework cations, wherein the zeolitecontains at most 1 proton per unit cell, and wherein the size, numberand charge of the extra-framework cations that are present in thezeolite are such that 1 or fewer non-proton extra-framework cations perunit cell are required to occupy 8-ring sites.

Aspect 2. The zeolite of Aspect 1, wherein the zeolite has a Si/Al ratioof from 3.6 to 4.2.

Aspect 3. The zeolite of Aspect 1 or 2, wherein the non-protonextra-framework cations comprise Li⁺, Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺and/or Zn²⁺ cations.

Aspect 4. The zeolite of any one of Aspects 1 to 3, wherein thenon-proton extra-framework cations comprise L⁺ and/or Zn²⁺ cations.

Aspect 5. The zeolite of Aspect 4, wherein said L⁺ and/or Zn²⁺ cationsmake up the majority of the extra-framework cations that are present perunit cell.

Aspect 6. The zeolite of Aspect 4 or 5, wherein said L⁺ and/or Zn²⁺cations make up at least 60% and more preferably at least 70% of theextra-framework cations that are present per unit cell.

Aspect 7. The zeolite of any one of Aspects 4 to 6, wherein said L⁺and/or Zn²⁺ cations make up at least 80% or at least 90% of theextra-framework cations that are present per unit cell.

Aspect 8. The zeolite of any one of Aspects 1 to 7, wherein the zeolitehas a unit cell axis length of from 14.23 Å to 14.55 Å.

Aspect 9. The zeolite of any one of Aspects 1 to 8, wherein the zeolitehas a unit cell axis length of from 14.23 Å to 14.50 Å.

Aspect 10. The zeolite of any one of Aspects 1 to 9, wherein the zeolitehas a unit cell axis length of from 14.30 Å to 14.45 Å.

Aspect 11. The zeolite of any one of Aspects 8 to 10, wherein thezeolite is selected from Li_(7.4)Zn_(1.2)RHO(3.9),Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9),Zn_(4.1)Li_(1.1)Na_(0.5)RHO(3.9), Zn_(5.7)RHO(3.2) and Zn_(4.9)RHO(3.9).

Aspect 12. The zeolite of any one of Aspects 1 to 7, wherein the zeolitehas a unit cell axis length of from 14.45 Å to 14.85 Å.

Aspect 13. The zeolite of Aspect 12, wherein the zeolite is selectedfrom Zn_(4.1)Li_(1.1) Na_(0.5)RHO(3.9) and Zn_(4.9)RHO(3.9).

Aspect 14. A process of adsorbing oxygen from a feed stream containingoxygen, comprising passing the feed stream through a bed of an adsorbentcomprising the RHO zeolite of any one of Aspects 8 to 11 to adsorboxygen from the feed stream, thereby producing a product stream depletedin oxygen.

Aspect 15. The process of Aspect 14, wherein the feed stream comprisesoxygen and one or both of nitrogen and argon, and the product stream isdepleted in oxygen and enriched in one or both of nitrogen and argon.

Aspect 16. The process of Aspect 14 or 15, wherein the process is a PSAprocess comprising an adsorption step performed at elevated pressure inwhich the feed stream is passed through a bed of adsorbent comprisingthe RHO zeolite of any one of Aspects 8 to 11 to adsorb oxygen from thefeed stream, and a desorption step performed at reduced pressure inwhich oxygen from the previous adsorption step is desorbed from the bedto regenerate the bed for the next adsorption step.

Aspect 17. A process of adsorbing nitrogen from a feed stream comprisingnitrogen and argon, comprising passing the feed stream through a bed ofan adsorbent comprising the RHO zeolite of Aspect 12 or 13 to adsorbnitrogen from the feed stream thereby producing a product streamdepleted in nitrogen and enriched in argon.

Aspect 18. The process of Aspect 17, wherein the process is a PSAprocess comprising an adsorption step performed at elevated pressure inwhich the feed stream is passed through a bed of adsorbent comprisingthe RHO zeolite of Aspect 12 or 13 to adsorb nitrogen from the feedstream, and a desorption step performed at reduced pressure in whichnitrogen from the previous adsorption step is desorbed from the bed toregenerate the bed for the next adsorption step.

Aspect 19. A process of adsorbing oxygen and nitrogen form a feed streamcomprising oxygen, nitrogen, and argon, comprising passing the feedstream through one or more beds of adsorbent comprising a firstadsorbent selective for nitrogen so as to adsorb nitrogen from the feedstream and a second adsorbent selective for oxygen so as to adsorboxygen from the feed stream, thereby producing a product stream enrichedin argon and depleted in oxygen and nitrogen, wherein the firstadsorbent comprises the RHO zeolite of Aspect 12 or 13 and/or whereinthe second adsorbent comprises the RHO zeolite of any one of Aspects 8to 11.

Aspect 20. The process of Aspect 19, wherein the second adsorbentcomprises the RHO zeolite of any one of Aspects 8 to 11.

Aspect 21. The process of Aspect 19 or 20, wherein the first adsorbenthas a Henry's law constant for nitrogen of from 0.5 to 3.0 mmole/gm/baraat 37.78° C.

Aspect 22. The process of any one of Aspects 19 to 21, wherein theprocess is a PSA process comprising an adsorption step performed atelevated pressure in which the feed stream is passed through a bed ofadsorbent comprising the first and second adsorbents to adsorb nitrogenand oxygen, respectively, thereby producing a product stream enriched inargon and depleted in oxygen and nitrogen, and a desorption stepperformed at reduced pressure in which oxygen and nitrogen from theprevious adsorption step are desorbed from the bed to regenerate the bedfor the next adsorption step.

Aspect 23. The process of Aspect 22, wherein the bed of adsorbentcomprises a first layer comprising the first adsorbent and a secondlayer comprising the second adsorbent, the first and second layers beingarranged such that during the adsorption step the feed stream passesthrough the first layer and contacts the first adsorbent for adsorptionof nitrogen before passing through the second layer and contacting thesecond adsorbent for adsorption of oxygen.

Aspect 24. A method of preparing a NaCsRHO zeolite having a desiredSi/Al ratio, comprising seeding an RHO preparation mixture with aNaCsRHO zeolite having the desired Si/Al ratio, and aging andcrystallizing the seeded RHO preparation mixture to form a RHO zeolitehaving a desired Si/Al ratio.

Aspect 25. The method of Aspect 24, wherein the RHO preparation mixtureis an aqueous mixture comprising 18-crown-6, water, and sources ofsilica, alumina, sodium, and cesium.

Aspect 26. A method of preparing a RHO zeolite containing Zn²⁺ and Li⁺cations, comprising providing a starting zeolite selected from a NaRHOor (NH₄)RHO or Na(NH₄)RHO zeolite, exchanging some or all of the Na^(+|)and/or NH₄ ⁺ cations in the starting zeolite with Zn²⁺ cations toprovide a zinc-exchanged RHO zeolite, and then exchanging some of thecations in the zinc-exchanged RHO zeolite to provide a RHO zeolitecontaining Zn²⁺ and L⁺ cations.

Aspect 27. The method of Aspect 26, wherein the step of providing thestarting zeolite comprises exchanging the Na⁺ and Cs⁺ cations in aNaCsRHO zeolite with NH₄ ⁺ cations to provide an (NH₄)RHO zeolite, andoptionally then exchanging some or all of the NH₄ ⁺ cations in the(NH₄)RHO zeolite with Na⁺ cations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of both O₂ uptake rate and kinetic selectivity of O₂vs. N₂ plotted against the level of proton exchange inLi_(9.0)Na_(0.8)RHO(3.9). ^(a)During full H⁺ exchange of RHO(3.9) someframework de-alumination occurs resulting in a Si/Al of 4.2, orH_(9.2)RHO(4.2)

FIG. 2 is a graph of both O₂ uptake rate and kinetic selectivity of O₂vs. Ar plotted against the level of proton exchange inLi_(9.0)Na_(0.8)RHO(3.9). ^(a)During full H⁺ exchange of RHO(3.9) someframework de-alumination occurs resulting in a Si/Al of 4.2, orH_(9.2)RHO(4.2).

FIG. 3 is a graph showing the variation in O₂ D/r² and O₂/Ar kineticselectivity with change in RHO unit cell axis for selected RHOcompositions.

FIG. 4a is a graph showing isotherm data measured up to 10 atm for O₂and Ar on Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) at three temperatures.

FIG. 4b is a graph showing isotherm data measured up to 1 atm for O₂ andAr on Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) at three temperatures.

FIG. 5 is a graph of the O₂ vs. N₂ equilibrium selectivity plottedagainst the level of proton exchange starting fromLi_(9.0)Na_(0.8)RHO(3.9). ^(a)During full H⁺ exchange of RHO(3.9) someframework de-alumination occurs resulting in a Si/Al of 4.2, orH_(9.2)RHO(4.2).

FIG. 6 is a graph of kinetic selectivity of O₂ vs. N₂ plotted against O₂uptake rate for a range of RHO and literature zeolites. ^(a)S. Farooq,Gas Separations and Purification, Vol. 9, No. 3, pp 205-212. ^(b)S.Kuznicki, B. Dunn, E Eyring, and D. Hunter, Separation Science andTechnology 2009, 44:7, pp 1604-1620.

FIG. 7 is a graph of kinetic selectivity of O₂ vs. Ar plotted against O₂uptake rate for a range of RHO and literature zeolites. ^(a)S. Farooq,Gas Separations and Purification, Vol. 9, No. 3, pp 205-212. ^(b)S.Kuznicki, B. Dunn, E Eyring, and D. Hunter, Separation Science andTechnology 2009, 44:7, pp 1604-1620.

FIG. 8 is a graph showing the isotherms of O₂ and N₂ at 5, 23, and 45°C. on Li_(6.8)H_(3.0)RHO(3.9).

FIG. 9 is a graph showing the isotherms of O₂ and N₂ at 5, 23, and 45°C. on Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9).

FIG. 10 is a graph showing the isotherms of O₂ and Ar at 5, 23, and 45°C. on Li_(9.5)Na_(1.6)Cs_(0.3)RHO(3.2).

FIG. 11 is a graph showing the isotherms of N₂ and Ar at 23 and 45° C.on Zn_(4.1)Na_(1.6)RHO(3.9).

FIG. 12 is a schematic illustrating the operation of a 2-bed multi-stepAr PSA cycle. Here F (F1, F2, and F3): feed, CoD: co-currentdepressurization, I (I1, I2, and I3): idle, EQD1: equalizationdepressurization 1, DEQD2: dual equalization depressurization 2, DEQD3:dual equalization depressurization 3, CnD (CnD1 and CnD2):counter-current depressurization, PU (PU1 and PU2): product purge, RP(RP1, RP2, and RP3): product re-pressurization, EQR1: top equalizationre-pressurization 1, DEQR2: dual equalization re-pressurization 2,DEQR3: dual equalization re-pressurization 3.

FIG. 13 depicts the results of a comparison of Ar recovery andproductivity on RHO and CMS type of adsorbent. The x-axis represents theconcentration of O₂ impurity in the product.

FIG. 14 depicts the results of a sensitivity study using a layeringconfiguration where N₂ selective equilibrium layer is placed at feed endfollowed by an O₂ selective kinetic layer at product end.

DETAILED DESCRIPTION

The ensuing detailed description provides preferred exemplaryembodiments only, and is not intended to limit the scope, applicability,or configuration of the invention. Rather, the ensuing detaileddescription of the preferred exemplary embodiments will provide thoseskilled in the art with an enabling description for implementing thepreferred exemplary embodiments of the invention, it being understoodthat various changes may be made in the function and arrangement ofelements without departing from the scope of the claimed invention.

The articles “a” and “an” as used herein mean one or more when appliedto any feature in embodiments of the present invention described in thespecification and claims. The use of “a” and “an” does not limit themeaning to a single feature unless such a limit is specifically stated.The article “the” preceding singular or plural nouns or noun phrasesdenotes a particular specified feature or particular specified featuresand may have a singular or plural connotation depending upon the contextin which it is used.

As used herein, “first,” “second,” “third,” etc. are used to distinguishfrom among a plurality of steps and/or features, and are not indicativeof the total number, or relative position in time and/or space unlessexpressly stated as such.

As used herein, the term “comprising” means consisting of or including.

As used herein, the phrase “and/or” placed between a first entity and asecond entity includes any of the meanings of (1) only the first entity,(2) only the second entity, and (3) the first entity and the secondentity. The term “and/or” placed between the last two entities of a listof 3 or more entities means at least one of the entities in the listincluding any specific combination of entities in this list. Forexample, “A, B and/or C” has the same meaning as “A and/or B and/or C”and comprises the following combinations of A, B and C: (1) only A, (2)only B, (3) only C, (4) A and B and not C, (5) A and C and not B, (6) Band C and not A, and (7) A and B and C.

As will be understood by those skilled in the art, zeolite structuresare often defined in terms of extended structural frameworks ofoxygen-linked tetrahedra (L. Pauling, “The Nature of the Chemical Bond”,3^(rd) Edition, Cornell University Press, Ithaca, 1960.; D. W. Breck,“Zeolite Molecular Sieves”, Robert E. Krieger Publishing Co., 1984). Inthe extended tetrahedral framework, the so-called “framework cations” ofthe structural framework (i.e. silicon, Si⁴⁺, and aluminum, Al³⁺,cations) are surrounded by oxygen anions, O²⁻, at the four corners of atetrahedron. When the charge of the framework cation is “4+”, as is thecase when the framework cation is the silicon cation, charge neutralityof the extended tetrahedral framework is maintained by sharing each ofthe oxygen anions between, on average, two tetrahedra, each tetrahedronbeing a charge neutral “SiO₄”. Conversely, where the charge of theframework cation is “3+”, as is the case when the framework cation isthe aluminum cation, similar sharing of oxygen anions between, onaverage, two tetrahedra leads to each aluminum-centered tetrahedronbeing negatively charged, i.e., “AlO₄ ⁻”. The resulting negative chargein the [(Al_(x)Si_(1-x))O₂]^(x−) polyanionic framework is balanced byso-called “extra-framework cations” that are located at interstitial ornon-framework sites in the extended structure. These charge-balancingextra-framework cations (which are often also called “non-frameworkcations”) can often be exchanged with other cations by standard cationexchange methods. Exemplary extra-framework cations include alkali andalkaline earth metal cations, transition metal cations, and protons(i.e. hydrogen cations).

As will also be understood by those skilled in the art, although theextra-framework cations and framework cations present in a zeolite arereferred to (as is conventional in the field) as cations, theirinteractions with the framework oxygen anions of the zeolite are not inpractice fully ionic. The bonding of the framework silicon and aluminumcations to the oxygen anions is sufficiently covalent that theseframework cations are not readily exchangeable with other cations bystandard cation exchange methods. As regards the extra-frameworkcations, it is likely that smaller, higher charge density cations suchas L⁺ or Ca²⁺, that create larger distortions in the RHO framework thanlarger, lower charge density cations such as K⁺ and Cs⁺, do so, in part,as a result of greater covalent interaction with framework oxygen ions.Also as regards the extra-framework cations, the hydrogen cation orproton may perform its role in balancing negative framework charge byforming a relatively covalent bond with the framework oxygen anion (andindeed it has been hypothesized that in practice said protons associatewith said oxygen anions in the form of structural hydroxyl groups; D. W.Breck, Zeolite Molecular Sieves, Robert E. Krieger Publishing Co.,1984).

As noted supra, RHO zeolites crystallize with a centrosymmetric bodycentered cubic (bcc) structure, and in their initially synthesized formcontain sodium and cesium cations. However, dehydration and/or cationexchange (i.e. substitution of the initially present sodium and cesiumcations with other extra-framework cations) can result in distortion ofthe cubic unit cell structure. As used herein, and unless otherwiseindicated, the term “RHO zeolite” without any further qualifierencompasses both RHO zeolites in their initially synthesized form andRHO zeolites that have been dehydrated and/or subject to cationexchange.

As used herein, the term “non-proton extra-framework cation” refers toany extra-framework cation that is not a proton (hydrogen cation).Unless otherwise indicated, all references to numbers of non-protonextra-framework cations that are present per unit cell indicate totalnumbers of non-proton extra-framework cations (of any and all types)rounded to one decimal place. Thus, a requirement that there are “1 orfewer non-proton extra-framework cations per unit cell that are requiredto occupy 8-ring sites” indicates that the maximum number, rounded toone decimal place, of non-proton extra-framework cations per unit cellthat may be required to occupy 8-ring sites is 1.0 non-protonextra-framework cations in total.

As used herein, the phrase “the zeolite contains at most 1 proton perunit cell” indicates that the zeolite contains at most 1.0 protons perunit cell (rounded to one decimal place), and so encompasses alsozeolites containing no protons, but excludes a zeolite containing 1.1protons or more per unit cell.

The extra-framework cation content of a zeolite, namely the numbers, andtypes, of extra-framework cations (including any protons) that arepresent per unit cell, can be determined by standard experimentaltechniques. For example, the extra-framework cation content of a zeolitecan be determined by elemental analysis of the solid by energydispersive spectroscopy (EDX) or by dissolution of a sample and analysisof the solution using inductively coupled plasma optical emissionspectroscopy (ICP-OES), as are also described in the Experimentalsection, infra. Where analysis of a zeolite by EDX or ICP-OES indicatesthat the negative charge per unit cell of the zeolite is not fullybalanced by the non-proton extra-framework cations identified as beingpresent, it is assumed that the remaining negative charge per unit cellis balanced by protons.

Unless otherwise indicated, all references herein to silicon to aluminum(Si/Al) ratios indicate Si/Al ratios rounded to one decimal place. TheSi/Al ratio of a zeolite can, for example, be determined using solidstate ²⁹Si NMR. Further details of suitable solid state ²⁹Si NMRtechniques and methods of determining the Si/Al ratio of a zeolite fromthe resulting NMR data are provided in the Experimental section, infra.

As used herein, the term “majority” means more than 50%. Thus, referenceherein to one or more cations making up the majority of theextra-framework cations that are present per unit cell of a RHO zeoliteindicates that said cation or cations, in their totality, constitutemore than 50% of all the extra-framework cations (including any protons)that are present per unit cell of the RHO zeolite.

As used herein, all references to percentages of cations that are ormust be present per unit cell indicate atomic percent (at. %) unlessotherwise indicated. Thus, reference herein to one or more cationsmaking up at least “X” % of the extra-framework cations that present perunit cell of a RHO zeolite indicates that said cation or cations, intheir totality, constitute equal to or greater than X at. % of all theextra-framework cations (including any protons) that are present perunit cell of the RHO zeolite. For example, in the zeoliteLi_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) there are 8 extra-frameworkcations per unit cell, of which 7 in total are L⁺ or Zn²⁺ cations; thusin this composition L⁺ or Zn²⁺ cations make up 87.5 at. % of theextra-framework cations that are present per unit cell, and so thiscomposition would meet a requirement that “at least 80% of theextra-framework cations that are present are L⁺ or Zn²⁺ cations.”

As used herein, all references to a zeolite using the designationRHO(X.X), where “X.X” is number, indicate a RHO zeolite having a siliconto aluminum ratio that is X.X (rounded to one decimal place). Thus, asnoted supra, the known prior art mixed-cation RHO zeoliteLi_(7.1)Na_(1.93)Cs_(0.3)Al_(11.7)Si_(36.3)O₉₆, which has a silicon toaluminum (Si/Al) ratio of 36.3/11.7=3.1025641, can also be referred toas Li_(7.1)Na_(1.93)Cs_(0.3)RHO(3.1).

Unless otherwise indicated, all references to the unit cell axis lengthof a RHO zeolite indicate the unit cell axis length of the RHO zeolitewhen dehydrated and as measured and determined using X-ray diffraction(XRD). Further details of suitable XRD techniques and methods ofdetermining the unit cell length axis of a RHO zeolite from theresulting XRD data are provided in the Experimental section, infra. Forexample, the unit cell axis length of a dehydrated sample of RHO zeolitecan be determined by Rietveld refinement against the XRD data. Unlessotherwise indicated, all references herein to unit cell axis lengths ofa RHO zeolite indicate the unit cell axis length rounded to two decimalplaces.

As is well known, pressure swing adsorption (PSA) processes comprise anadsorption step which the feed stream containing the substance orsubstances to be adsorbed is passed through a bed of adsorbent at anelevated pressure, and a desorption step in which substances adsorbed inthe previous adsorption step are desorbed from the bed at reducedpressure. In this context, the terms “elevated pressure” and “reducedpressure” refer only to the relative pressures in the bed during the twosteps, i.e. the pressure during the adsorption step is elevated relativeto the pressure during the desorption step but may otherwise be of anysuitable pressure for carrying out the adsorption step, and the pressureduring the desorption step is reduced relative to the pressure duringthe adsorption step but may otherwise be of any suitable pressure forcarrying out the desorption steps. Suitable operating pressures andcycles for carrying out PSA are well known to those of ordinary skill inthe art.

As used herein, the term “RHO preparation mixture” refers to any mediumthat can be aged and crystallized to form a RHO zeolite, and includeswithin its meaning solutions, gels and dispersions. Where thepreparation mixture is an aqueous mixture comprising 18-crown-6, waterand sources of silica, alumina, sodium and cesium, sources of silica,alumina, sodium and cesium may be of any suitable type, such as thosewell known in the art. For example, colloidal silica may be used as thesource of silica, and aluminum isopropoxide and/or sodium aluminate maybe used as sources of alumina, and sodium hydroxide and cesium hydroxidemay be used as sources of sodium and cesium, respectively. Furthersuitable sources are described in U.S. Pat. Nos. 3,904,738 and7,169,212, for example.

As used herein, and unless otherwise indicated, a “NaRHO” zeolite is aRHO zeolite in which the only extra-framework cations are sodiumcations, a “(NH₄)RHO” zeolite is a RHO zeolite in which the onlyextra-framework cations are ammonium cations, a “Na(NH₄)RHO” zeolite isa RHO zeolite in which the only extra-framework cations are sodium andammonium cations, a “NaCsRHO” zeolite is a RHO zeolite in which the onlyextra-framework cations are sodium and cesium cations, and so on.

Disclosed herein are RHO zeolites having a Si/Al ratio of from 3.2 to4.5 and containing non-proton extra-framework cations, wherein thezeolite contains at most 1 proton per unit cell, and wherein the size,number and charge of the extra-framework cations that are present in thezeolite are such that 1 or fewer non-proton extra-framework cations perunit cell are required to occupy 8-ring sites

These RHO compositions demonstrate improved productivity, capacity,and/or regenerative properties at ambient (0 to 50° C.) and sub-ambient(<0° C.) temperatures that make them suitable for Ar or N₂ PSAapplications as well as methane upgrading, i.e., CO₂ removal frommethane, applications.

Without intending to be bound by theory, it is believed that the RHOzeolites of this invention achieve their desirable adsorption propertiesbased on the nature of the 8-ring openings of these structures, throughwhich gas molecules must pass to enter the RHO cages. In RHO, theserings are very flexible and can undergo significant distortion fromcircular to highly elliptical depending on extra framework cation siteand type. Optimization of the extra framework cation sites, as well asthe generation of elliptical 8-ring openings, are likely importantfactors in allowing the very rapid uptake of elongated oxygen moleculesversus the slow uptake of spherical argon atoms.

Extra-framework cations in RHO zeolites can occupy at least one of threedifferent sites in the unit cell cage. It is known that small cations,comparable in size to Li⁺, i.e. those with a 6-coordinate cation radiusas defined by Shannon (R. D. Shannon, Acta Cryst. 1976, A32, 751-767.)of 0.8 Å and below, can reside in 6-ring openings within the unit cellcage. There are 8 of these sites per RHO unit cell in any RHOcomposition with Si/Al of 5 or less. Smaller cations, 0.8 Å and below,can also occupy one of 12 sites at the center of individual 8-ringopenings, and will be required to occupy those sites if all eight of the6-ring sites are already filled. In addition to Li⁺, examples of cationswith 6-coordinate Shannon cationic radii smaller than 0.8 Å are Mg²⁺ anddivalent cations of the first-row transition series and morespecifically Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu^(2+,) and Zn²⁺.

Intermediate size cations, i.e. those with a 6-coordinate Shannon cationradius from 0.8 to approximately 1.3 Å, can reside in one of 12 sites atthe center of individual 8-ring openings and, in some cases, at one of 6sites at the center of two 8-ring windows in the RHO unit cell. They areunable to fit in the 6-ring sites under ambient conditions, i.e. between0 and 50° C. As an example, in U.S. Pat. No. 5,944,876, Corbin teachesof fully and partially Cd exchanged RHO zeolites, with Si/Al>3,including RHO compositions with at least 1 Cd²⁺ cation per unit cell,with an assortment of other cations. Because of the size of the Cd²⁺cations these compositions require at least one cation, namely the Cd²⁺cation to reside in an 8-ring position.

The largest cations with a 6-coordinate Shannon cation radius largerthan 1.3 Å, including Cs⁺, which is required in the RHO crystallizationprocess, occupy one of 6 sites at the center of two 8-ring windows inthe RHO unit cell.

Small, monovalent cations, such as L⁺ are very electropositive and havebeen shown to cause large elliptical distortions in the 8-ring openings.Larger, divalent cations, such as Ca²⁺ and Cd²⁺ are also veryelectropositive and have been shown to cause even larger distortions ofthe 8-ring openings. In contrast, very small protons, or H⁺ cations,cause no distortion of the RHO 8-rings, presumably because they directlybind to one of the zeolite oxygen atoms.

It has been suggested that while the larger Ca²⁺ cations distort andblock 8-ring openings, thereby inhibiting gas uptake, the smaller L⁺cations, while still distorting the 8-rings, could leave enough of the8-rings open to still permit some gas uptake. This concept wasdemonstrated, in part, by Corbin in U.S. Pat. No. 7,169,212, who showedthat Li_(7.1)Na_(1.93)Cs_(0.3)RHO(3.1) could adsorb O₂ with effectiveexclusion of N₂. Unfortunately, the O₂ uptake rate of this material isextremely slow and is too slow for PSA applications. While no detailedstructural data with cation positions is presented for this material,one can infer that at least 2.23 of the non-proton cations must occupy8-ring blocking positions. The sodium and cesium cations are too largeto fit in 6-rings and fill a total of 2.23 of the 8-ring positions perunit cell. The full cation balance is not reported for this material,and it is possible that additional non-proton cations are forced toreside in the 8-ring. If the cation balance was completed with anymonovalent cations other than protons, as many as 3.7 non-proton cationswould be required to reside in 8-rings. Regardless, the O₂ uptake ratereported is very slow and is consistent with at least 2.2 non-protoncations being forced to reside in the 8-rings of this composition, basedon other comparative examples.

As a comparative example Li_(9.5)Na_(1.6)Cs_(0.3)RHO(3.2) was prepared,which nominally contains the same number of cations/unit cell as thematerial reported by Corbin, and was targeted to achieve the same numberof Na⁺ and Cs⁺ cations. In this composition, one can infer that at least3.4 of the cations must occupy 8-ring blocking positions. The sodium andcesium cations are too large to fit in 6-rings and fill a total of 1.9of the 8-ring positions per unit cell. Of the 9.5 L⁺ cations per unitcell, once all 8 of the 6-ring positions are filled, 1.5 must go in8-ring positions, making a total of 3.4 cations in 8-ring positions inthis unit cell. This material showed nitrogen exclusion behavior likethe above RHO composition of Corbin, and the oxygen uptake rate observedwas exceedingly slow.

For a RHO material with Si/Al of 3.2, even when all of the cations aresmall enough to fit in 6 ring windows, if they are monovalent, such asLi⁺, 3.4 of them would be required to reside in 8-ring windows, based onthe charge balance required for a RHO(3.2) material. In a comparativeexample, described infra, fully lithium exchanged Li_(11.4)RHO(3.2) hasbeen demonstrated to show good kinetic selectivity for oxygen overnitrogen and argon, but the oxygen uptake rate is still much slower thanCMS and indeed is comparable to the material described by Corbin.

In other comparative examples, described infra, it has been shown thatfully proton exchanged H_(10.7)RHO(3.5) and H_(9.2)RHO(4.2) adsorb O₂,N₂, and Ar very rapidly, but non-selectively, consistent with an absenceof distortion in the 8-ring windows.

For RHO compositions with Si/Al between 3.2 and 4.5, the extent ofaluminum substitution, and consequently the formal negative charge whichmust be balanced, ranges from 11.4 to 8.7. In the novel RHO zeolites ofthis invention, <1 of the extra-framework cations per unit cell is aproton and, additionally, the extra-framework cations are chosen suchthat 1 or fewer non-proton extra-framework cations are forced to reside,by virtue of their size, charge, and/or the total number ofextra-framework cations, in 8-ring blocking sites. For example, aspreviously discussed, Li_(11.4)RHO(3.2) is forced to have at least 3.4L⁺ cations in 8-ring blocking positions, once all 6-ring positions arefilled. A new Rho zeolite composition of this invention,Zn_(5.7)RHO(3.2), satisfies the charge balance of RHO(3.2) withpotentially no cations in 8-ring windows, i.e., all cations are smallenough to fit in 6-rings (the Shannon 6-coordinate cation radius forZn²⁺ is 0.74 Å), and there are enough 6-rings to hold all of thecations. In actuality, some of the Zn²⁺ cations are observed to residein 8-rings in the Zn_(5.7)RHO(3.2) unit cell but, by virtue of theirsize and number, there is the potential for them to move between the 6and 8 rings. The Zn_(5.7)RHO(3.2) composition shows excellent kineticselectivity for O₂ vs. N₂ and O₂ vs. Ar, and its O₂ D/r² is 900 timesfaster than that of Li_(11.4)RHO(3.2), 915 times faster than that ofLi_(9.5)Na_(1.6)Cs_(0.3)RHO(3.2), and 10 times faster than an exampleCMS material. This composition as well as other new RHO zeolitecompositions of this invention, includingZn_(4.1)Li_(1.1)Na_(0.5)RHO(3.9) and Zn_(4.9)RHO(3.9), also showexcellent equilibrium selectivity for N₂ over argon, making thempotentially useful for removal of trace N₂ from argon. The larger unitcell composition Zn_(4.1)Na_(1.6)RHO(3.9) also shows excellentequilibrium selectivity for N₂ over argon, but shows lower kineticselectivity for O₂ vs. Ar than some of the compositions of thisinvention.

In addition to choosing small, divalent cations, the number ofdistorting cations required to be in 8-ring windows can also bedecreased by increasing the Si/Al ratio. As the Si/Al ratio of RHO goesfrom 3.2 to 3.9 to 4.2, LiRHO compositions go from Li_(11.4)RHO(3.2), toLi_(9.8)RHO(3.9), to Li_(9.2)RHO(4.2). In the present examples, theknown composition, Li_(9.8)RHO(3.9), was found to contain low levels ofsodium, and actually has the composition Li_(9.0)Na_(0.8)RHO(3.9) by ICPanalysis. Required 8-ring occupancy in this series goes from 3.4 to 1.8to 1.2. O₂ D/r² increases from 1 to 67 to 2400 relative to that ofLi_(11.4)RHO(3.2), while O₂ vs. N₂ and Ar kinetic selectivities remainhigh for all. For oxygen removal, both the Li_(9.0)Na_(0.8)RHO(3.9)(nominally Li_(9.8)RHO(3.9)) composition and the Li_(9.2)RHO(4.2)composition are potentially useful. Both show very high kineticselectivity for O₂ vs. Ar, while the first composition has an O₂ D/r²uptake comparable to that of CMS and the second composition has an O₂D/r² uptake rate 27 times faster than CMS.

A third way to reduce the number of distorting cations required tooccupy blocking 8-ring positions involves the substitution of blockingcations with protons, which presumably do not distort the 8-rings.Corbin in U.S. Pat. No. 7,169,212, mentions the possibility that mixedcation RHO materials with partial exchange of H⁺ (e.g. H,CsRHO) could beprepared which would “give at least some of the desired distortion andsmaller pore size,” but no specific compositions were reported.Recently, Paul A. Wright and co-workers (J. Am. Chem. Soc. 2012, 134,17628) described the preparation of mixed Li_(9.8-x)H_(x)RHO(3.9), wherex is at least 1.4, but no adsorption data was reported. In the presentapplication a number of Li_(9.8-x)H_(x)RHO(3.9) compositions have beensuccessfully prepared, and are described infra, in which the number ofnon-proton cations forced to reside in 8-ring blocking sites have beenlowered to 1 or fewer, similar to the new RHO zeolite compositions ofthis invention. Unless otherwise indicated, all adsorption data, bothequilibrium and kinetic, was measured at 30° C. As shown in FIGS. 1 and2, substitution of L⁺ cations in Li_(9.0)Na_(0.8)RHO(3.9) with varyingnumbers of protons leads to a dramatic increase in O₂ D/r² (up to 40times higher than Li_(9.0)Na_(0.8)RHO(3.9)). Surprisingly, good O₂ vs.N₂ and O₂ vs. Ar kinetic selectivity is maintained to up to compositionsof Li_(5.8)H₄RHO(3.9) and H₆Li_(5.4)RHO(3.2). Results of Ar PSAsimulations on Li_(6.8)H₃RHO(3.9) shown in Table 4 and Example 11, showa large improvement in Ar recovery and productivity over CMS materials,though not quite as high one of the new RHO zeolite compositions of thisinvention, Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9).

A comparison of the unit cell axis data in Table 3 and FIG. 3 with theO₂ D/r² and O₂/Ar kinetic selectivity data show the optimal RHO zeolitematerials for rapid separation of O₂ from Ar. These are RHOcompositions, having 1.8 or fewer and preferably 1.6 or fewer and mostpreferably 1.0 or fewer non-proton extra-framework cations per unitcell, which, by virtue of their size, charge and/or number, are requiredto reside in 8-ring sites; and having a cubic or approximately cubicunit cell defined by a unit cell axis length between 14.23 and 14.55 Å,more preferably between 14.23 and 14.50 Å, and most preferably between14.30 and 14.45 Å. Those with a smaller unit cell axis than 14.23 Å,including Li_(11.4)RHO(3.2), show slower O₂ uptake than typical CMSmaterials, while those with larger unit cell axes than 14.55 Å, likeH_(9.2)RHO(4.2), show lower kinetic selectivity for O₂ over Ar than CMSmaterials. Several of the new RHO zeolite compositions of this inventionwith larger unit cell axes, from 14.45 Å to 14.85 Å, additionally oralternatively show potential for the rapid separation of trace N₂ fromAr by an equilibrium separation process. A comparison of the unit celland uptake data in Table 3 shows that both Zn_(4.9)RHO(3.9) (with unitcell axis=14.54 Å) and Zn_(4.1)Li_(1.1)Na_(0.5)RHO(3.9) (unit cellaxis=14.49 Å) rapidly adsorb N₂ with equilibrium selectivity of N₂ vs.Ar of approximately 3. FIG. 11 shows the isotherm data forZn_(4.1)Na_(1.6)RHO(3.9) confirming high N₂ vs Ar equilibriumselectivity over a range of pressures and temperatures. It is likelythat these compositions would also be useful for other rapid separationsincluding that of CO₂ from methane.

A number of RHO zeolite compositions described herein provide highkinetic selectivity for O₂ versus Ar and O₂ versus N₂ adsorption atambient temperatures. As shown in FIG. 4b , an O₂ versus Ar equilibriumselectivity close to 1 is observed onLi_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) at 23° C. Effective equilibriumselectivity for O₂ versus Ar is observed as the adsorption temperatureis dropped below 23° C., which is likely associated with temperaturedependent contraction of the RHO 8 ring windows as well as reducedvibration of the rings. Thus, the RHO zeolite materials of thisinvention appear to have ideal properties for removal of O₂ from argoncontaining streams.

The O₂ vs. N₂ equilibrium selectivity of low-silica zeolites istypically less than 0.5, due to the stronger quadrupole interactionsbetween N₂ and electropositive extra framework cations of the zeolite.The RHO zeolites also show this behavior, but as can be seen in FIG. 5,the O₂ vs. N₂ equilibrium selectivity can be significantly improved byincreasing the number of protons exchanged for Li⁺ in aLi_(9.0)Na_(0.8)RHO(3.9) zeolite. Complete exchange to the fullyprotonated RHO increases the O₂/N₂ equilibrium selectivity to 0.76.Unfortunately, the lack of 8-ring distortion by these cations leads tonegligible kinetic selectivity between O₂ and N₂. By balancing improvedO₂ vs. N₂ equilibrium selectivity with high kinetic selectivity andrate, it appears that an optimum may in some instances be achieved onexchange of 3 to 4 protons (FIGS. 1 and 5).

Also described herein are processes, and in particular PSA processes, ofadsorbing oxygen from a feed stream containing oxygen (such as forexample air or a crude argon feed stream), comprising passing the feedstream through a bed of an adsorbent comprising a RHO zeolite to adsorboxygen from the feed stream, thereby producing a product stream depletedin oxygen, wherein the RHO zeolite is a RHO zeolite having a Si/Al ratioof from 3.2 to 4.5 and containing non-proton extra-framework cations,wherein the zeolite contains at most 1 proton per unit cell, wherein thesize, number and charge of the extra-framework cations that are presentin the zeolite are such that 1 or fewer non-proton extra-frameworkcations per unit cell are required to occupy 8-ring sites, and whereinthe zeolite has a unit cell axis length of from 14.23 Å to 14.55 Å.

Surprisingly, the present inventors have observed that many of the novelRHO zeolite compositions described herein are suitable for rapid O₂separation from Ar, provided the RHO zeolite composition has anapproximately cubic unit cell with a unit cell axis length from 14.23 to14.55 Å. These compositions are well suited for use in pressure swingadsorption techniques at ambient (0° C. to 50° C.) temperatures and itis believed that they would be effective at cold temperatures in theprocess described by U.S. patent application Ser. No. 15/049,704, METHODFOR ARGON PRODUCTION VIA COLD PRESSURE SWING ADSORPTION (U.S. Pat. No.9,708,188).

Also described herein are processes, and in particular PSA processes, ofadsorbing nitrogen from a feed stream comprising nitrogen and argon(such as for example air or a crude argon feed stream), comprisingpassing the feed stream through a bed of an adsorbent comprising the RHOzeolite to adsorb nitrogen from the feed stream thereby producing aproduct stream depleted in nitrogen and enriched in argon, wherein theRHO zeolite is a RHO zeolite having a Si/Al ratio of from 3.2 to 4.5 andcontaining non-proton extra-framework cations, wherein the zeolitecontains at most 1 proton per unit cell, wherein the size, number andcharge of the extra-framework cations that are present in the zeoliteare such that 1 or fewer non-proton extra-framework cations per unitcell are required to occupy 8-ring sites, and wherein the zeolite has aunit cell axis length of from 14.45 Å to 14.85 Å.

Several of the novel RHO zeolite compositions of this invention withunit cell axis from 14.45 Å to 14.85 Å show potential for the rapidremoval of trace N₂ from Ar by an equilibrium separation process. BothZn_(4.9)RHO(3.9) (with unit cell axis=14.54 Å) andZn_(4.1)Li_(1.1)Na_(0.5)RHO(3.9) (unit cell axis=14.49 Å) show rapiduptake of N₂ with equilibrium selectivity of N₂ vs. Ar of 3.

Also described herein are processes, and in particular PSA processes, ofadsorbing oxygen and nitrogen form a feed stream comprising oxygen,nitrogen, and argon (such as for example air or a crude argon feedstream), comprising passing the feed stream through one or more beds ofadsorbent comprising a first adsorbent selective for nitrogen so as toadsorb nitrogen from the feed stream and a second adsorbent selectivefor oxygen so as to adsorb oxygen from the feed stream, therebyproducing a product stream enriched in argon and depleted in oxygen andnitrogen, wherein the first adsorbent comprises a RHO zeolite of thepresent invention having a unit cell axis length from 14.45 to 14.85 Åand/or wherein the second adsorbent comprises a RHO zeolite of thepresent invention having a unit cell axis length of from 14.23 Å to14.55 Å. In such processes, it is particularly preferred that the secondadsorbent comprises a RHO zeolite of the present invention having a unitcell axis length from 14.23 to 14.55 Å; and first adsorbent comprises aRHO zeolite of the present invention having a unit cell axis length offrom 14.45 Å to 14.85 Å and/or is an adsorbent having a Henry's lawconstant for nitrogen of from 0.5 to 3.0 mmole/gm/bara at 37.78° C.

Also described herein is a method of preparing a NaCsRHO zeolite havinga desired Si/Al ratio, comprising seeding an RHO preparation mixturewith a NaCsRHO zeolite having the desired Si/Al ratio, and aging andcrystallizing the seeded RHO preparation mixture to form a RHO zeolitehaving a desired Si/Al ratio.

The inventors have found that such methods provide a convenient way ofpreparing the RHO zeolite compositions, including but not limited to thenovel RHO zeolites described herein, using reduced levels of templatingagent. As previously described by Chatelain in (Microporous Materials,1995, 4, 231), RHO(3.9) can be readily prepared using 18-crown-6 as atemplating or structure directing agent. While effective, 18-crown-6 isexpensive, and the literature preparation uses this reagent inapproximately stoichiometric amounts with CsOH. The present applicationdescribes a process whereby seeding the RHO preparation gel compositionwith approximately 10 wt. % of Na_(6.8)Cs_(3.0)RHO(3.9) (relative to theamount of RHO(3.9) product produced) allows the quantity of 18-crown-6used to be cut by 80%. Use of small amounts of Na_(8.4)Cs_(3.0)RHO(3.2)seed material also allows RHO(3.2) to be prepared much more reliably.While as synthesized Na_(8.4)Cs_(3.0)RHO(3.2) andNa_(6.8)Cs_(3.0)RHO(3.9) are used in seeding the targeted RHOpreparations, it is believed that any RHO material with the target Si/Alwould be effective.

Also described herein is a method of preparing a RHO zeolite containingZn²⁺ and L⁺ cations, comprising providing a starting zeolite selectedfrom a NaRHO or (NH₄)RHO or Na(NH₄)RHO zeolite, exchanging some or allof the Na⁺ and/or NH₄ ⁺ cations in the starting zeolite with Zn²⁺cations to provide a zinc-exchanged RHO zeolite, and then exchangingsome of the cations in the zinc-exchanged RHO zeolite to provide a RHOzeolite containing Zn²⁺ and L⁺ cations.

When preparing mixed cation RHO zeolite compositions, particularly thosecontaining L⁺ and Zn²⁺ cations, the present inventors have observed thatorder of cation exchange significantly impacts the loading of Zn²⁺achievable in the zeolite. This can be seen from the exchange data inTable 1. When starting with Li_(11.4)RHO(3.2), a single Zn-exchangewith >100:1 molar ratio of 2M Zn(NO₃)₂ exchange solution concentrationto zeolite concentration leads to the replacement of only 2.4 of the11.4 L⁺ cations. A second exchange under the same conditions replacesonly one additional L⁺ cation. Complete replacement of L⁺ with Zn²⁺ isapparently a very slow and difficult process, perhaps because most ofthe Li⁺ cations must be exchanged from very small 6-ring sites. Incontrast exchanging Zn²⁺ into RHO appears to proceed much more easilystarting from the sodium exchanged RHO. Only 3 exchanges, using only a40:1 molar ratio of 1.5 M Zn(NO₃)₂ exchange solution concentration tozeolite concentration, are required to fully load the RHO structure withZn²⁺ cations. Back-exchange of the resulting Zn_(5.7)RHO(3.2) orZn_(4.9)RHO(3.9) composition with L⁺ appears to proceed smoothly.

EXAMPLES

The compositions described herein in the following examples werecharacterized in the following manner. In addition to measuring theiradsorption properties, novel zeolite compositions were characterized byX-ray diffraction, ²⁹Si NMR, by elemental analysis using ICP-OES, and byscanning electron microscopy (SEM).

Powder x-ray diffraction (XRD) patterns of hydrated and dehydratedsamples were measured in Debye-Scherrer geometry on a Stoe STAD i/pdiffractometer with monochromated Cu Kα₁ X-rays (λ=1.54056 Å). Inaddition, laboratory powder X-ray diffraction for Rietveld refinementwas performed on samples in quartz glass capillaries that had beenactivated at the glass line at 623 K for 10 hours and sealed using ablow-torch. The unit cell axis length and the number, and position ofextra framework cations for Li,M-Rho samples were determined by Rietveldrefinement against the laboratory PXRD data, using the GSAS suite ofprograms.

Solid state ²⁹Si NMR spectra were obtained at ambient temperature on aBruker Avance II 300 FT-NMR spectrometer, equipped with a 7 mm MASprobe. The acquisition was carried out using one pulse employing an8-second recycle delay while the rotor was spun at 5000 Hz at magicangle. Peak deconvolution was performed using GRAMS/32 Al (version 6.00)software. Mixed Gaussian/Lorentzian line shapes were employed. From therelative peak areas, the Si/Al ratio was calculated using Equation 1.

$\frac{Si}{Al} = \frac{\sum\limits_{n = 0}^{4}I_{{Si}_{nAl}}}{\sum\limits_{n = 0}^{4}{0.25\; n\mspace{14mu} I_{{Si}_{({nAl})}}}}$

where: Si/Al=Silicon to Aluminum ratio, I=Relative area of NMR peak.

Si_((nAl))=Silicon with n aluminum atoms as nearest neighbor boundthrough oxygen.

n=Number of nearest aluminum atoms represented by the NMR peak.

The cation exchange level was determined by elemental analysis usingestablished methods, which involved either direct analysis on the solidzeolite by energy dispersive spectroscopy (EDX) or dissolution of thesolid and subsequent analysis of the solution using inductively coupledplasma optical emission spectroscopy (ICP-OES).

A Perkin Elmer 3000DV Inductively Coupled Plasma Optical EmissionSpectrometer was used for the sample analysis.

SEM analyses were performed using a Hitachi S-4800 field-emission SEMoperated at 2 kV accelerating voltage.

EDX analysis on all samples was performed in a JEOL JSM 5600 SEM, withan Oxford INCA Energy 200 EDX analyser.

Example 1: Synthesis of Na_(8.4)Cs_(3.0)RHO(3.2)

The modified RHO synthesis method described by Corbin et al. in U.S.Pat. No. 7,169,212, was used to prepareNa_(8.4)Cs_(3.0)Al_(11.4)Si_(36.6)O₉₆, or Na_(8.4)Cs_(3.0)RHO(3.2), aRHO zeolite composition with nominal Si/Al=3.2. The purity of thecrystalline material was determined by XRD and Si/Al by ²⁹Si NMR. Bothmatched reported data for the hydrated form of RHO(3.2) zeolites. SEMimages show an average particle size of 2.5 μm.

Example 2: Synthesis of Na_(6.8)Cs_(3.0)RHO(3.9)

RHO(3.9) was prepared by the method of Chatelain et al. (MicroporousMaterials, 1995, 4, 231). Crystallization and isolation followed bycalcination in air at 550° C. gives Na_(6.8)Cs_(3.0)Al_(9.8)Si_(38.2)O₉₆or Na_(6.8)Cs_(3.0)RHO(3.9). The purity of the crystalline material wasdetermined by XRD and the Si/Al ratio by ²⁹Si NMR. Results from bothmethods matched reported data for the hydrated form of RHO(3.9)zeolites. SEM images show an average particle size of ˜1 μm.

Example 3: Synthesis of RHO Zeolites with Reduced Levels of TemplatingAgent Na_(8.4)Cs_(3.0)RHO(3.2)

Aluminum isopropoxide (3.67 g) was dissolved in aqueous NaOH solution(50 weight %, 4.5 g) at 100° C. and left to cool to 25 CC. Aqueous CsOHsolution (50 weight %, 2.7 g) was then added with stirring, followed by18-crown-6 (0.27 g). Next, Na_(8.4)Cs_(3.0)RHO(3.2) was added as a seedmaterial (0.6 g), followed by colloidal silica (Ludox 30, 20 g), andthen, distilled water (0.35 g). Once homogeneous, the gel was aged at25° C. for 4 days before crystallizing in a polypropylene bottle at 90°C. for 5 days under static conditions. The resulting white solid wasfiltered and washed before being dried at 90° C. overnight. The productwas placed in a ceramic dish and slowly heated in a Fisher Scientificmuffle furnace under 10 L/min ambient air purge to 300° C. at 0.4°C./min, and then heated to 550° C. at 1° C./min. Calcination of theproduct continued under 10 L/min ambient air purge at 550° C. for 24hours. Approximately 6 g of pure RHO(3.2) product was obtained.

Na_(6.8)Cs_(3.0)RHO(3.9):

NaOH (0.59 g) and 18-crown-6 (0.27 g) were dissolved in CsOH solution(50 weight %, 1.8 g) and distilled water (0.78 g) before adding sodiumaluminate (1.82 g) and stirring until homogenous.Na_(6.8)Cs_(3.0)RHO(3.9) was added as seed material (0.6 g), followed bycolloidal silica (Ludox 40, 15 g), and the mixture was stirred untilhomogenous. The gel was aged at 25° C. for 4 days before crystallizingin a polypropylene bottle at 90° C. for 5 days under static conditions.The resulting white solid was filtered and washed before being dried at90° C. overnight. The product was placed in a ceramic dish and slowlyheated in a Fisher Scientific muffle furnace under 10 L/min ambient airpurge to 300° C. at 0.4° C./min, and then heated to 550° C. at 1°C./min. Calcination of the product continued under 10 L/min ambient airpurge at 550° C. for 24 hours. Approximately 6 g of pure RHO(3.9) wasobtained.

Example 4: Cation Exchange of RHO Zeolites

A variety of exchanged RHO(3.2 to 4.2) materials were prepared throughion exchange of the starting Na_(8.4)Cs_(3.0)RHO(3.2) andNa_(6.8)Cs_(3.0)RHO(3.9) from Examples 1 through 3, as well asH_(9.2)RHO(4.2) from Example 6. Ammonium exchanged RHO samples wereprepared by repeated (8 times) exchange with a 40-fold excess (mole %basis) of 1M ammonium chloride solution at 90° C. for at least 4 hrs.Sodium exchanged RHO materials were prepared from ammonium RHO zeolitesthrough repeated (8 times) exchange with a 40-fold excess (mole % basis)of 1M sodium chloride solution at 90° C. for at least 4 hrs. Theresulting sodium RHO could be readily exchanged with Zn²⁺ or Cu²⁺ usingexcess 1.5 M Zn(NO₃)₂ or 1.5M Cu(NO₃)₂ solutions at 90° C. Usually, twoexchange steps were carried out to ensure complete exchange. Exchange ofNaRHO to the LiRHO forms was more difficult and required at least 6exchanges with 1M lithium chloride at 90° C. for at least 4 hrs.Exchange of LiRHO materials with Zn²⁺ using Zn(NO₃)₂ solutions was verydifficult to drive to completion and typically only exchanged 1 to 1.5Zn²⁺ cations per unit cell into the LiRHO composition. Final exchangecompositions were determined by ICP-OES or EDX. Where the analysis givesa cation charge balance which is lower than that needed for the numberof aluminum atoms/unit cell in a given RHO composition, the differenceis assumed to be made up with protons, e.g.Li_(5.2)Zn_(1.8)Na_(0.5)RHO(3.9) is adjusted toLi_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) to fully balance the charge ofthe 9.8 alumina centers/unit cell. A number of compositions and exchangeconditions are shown in Table 1.

TABLE 1 Exchange Conditions for Pure and Mixed Cation RHO SamplesSalt/zeolite Contact Exchange Exchange Starting RHO Salt Solution (molarratio) Time (hrs) Temperature ° C. Repeats Product CompositionNa_(8.4)Cs_(3.0)RHO(3.2) 1M NH₄Cl 40 >4 90 8 (NH₄)_(11.4)RHO(3.2)(NH₄)_(11.4)RHO(3.2) 1M NaCl 40 >6 90 8 Na_(11.4)RHO(3.2)Na_(11.4)RHO(3.2) 2M Zn(NO₃)₂ 40 >4 90 4 Zn_(5.7)RHO(3.2)Na_(11.4)RHO(3.2) 1.5M Cu(NO₃)₂ 30 >4 90 4 Cu_(5.7)RHO(3.2)Na_(11.4)RHO(3.2) 1M LiCl 40 >4 90 8 Li_(11.4)RHO(3.2)Na_(6.8)Cs_(3.0)RHO(3.9) 1M NH₄Cl 40 >4 90 8 (NH₄)_(9.8)RHO(3.9)(NH₄)_(9.8)RHO(3.9) 1M NaCl 40 >6 90 8 Na_(9.8)RHO(3.9)(NH₄)_(9.8)RHO(3.9) 2M Zn(NO₃)₁ >100 >4 60 4 Zn_(4.9)RHO(3.9)Na_(9.8)RHO(3.9) 2M Zn(NO₃)₂ 40 >4 90 3 Zn_(4.1)Na_(1.6)RHO(3.9)Na_(9.8)RHO(3.9) 1.5M Cu(NO₃)₂ 30 >4 90 4 Cu_(4.9)RHO(3.9)Na_(9.8)RHO(3.9) 1M LiCl 40 >4 90 8 Li_(9.0)Na_(0.8)RHO(3.9)Li_(9.0)Na_(0.8)RHO(3.9) 0.07M NH₄Cl 2 >4 90 1 Li_(7.8)(NH₄)₂RHO(3.9)Li_(9.0)Na_(0.8)RHO(3.9) 0.1M NH₄Cl 3 12 90 1 Li_(6.8)(NH₄)₃RHO(3.9)Li_(9.0)Na_(0.8)RHO(3.9) 0.1M NH₄Cl 4 12 90 1 Li_(5.8)(NH₄)₄RHO(3.9)Li_(11.4)RHO(3.2) 2M Zn(NO₃)₂ >100 >4 60 1 Li_(9.0)Zn_(1.2)RHO(3.2)Li_(9.0)Zn_(1.2)RHO(3.2) 2M Zn(NO₃)₂ >100 >4 60 1Li_(8.0)Zn_(1.7)RHO(3.2) Li_(9.2)H_(0.6)RHO(3.9) 2M Zn(NO₃)₂ >100 0.5 601 Li_(7.4)Zn_(1.2)RHO(3.9) Zn_(4.1)Na_(1.6)RHO(3.9) 1M LiCl 30 12 90 1Zn_(4.1)Li_(1.1)Na_(0.5)RHO(3.9) Zn_(4.1)Li_(1.1)Na_(0.5)RHO(3.9) 1MLiCl 30 12 90 1 Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9)

Example 5: Synthesis of H_(10.7)RHO(3.5)

Na_(8.4)Cs_(3.0)RHO(3.2), from Example 1 was mixed with a 10-fold excess(mole % basis) of 1M ammonium chloride solution at 90° C. for at least 4hrs. After mixing, the material was filtered. The ammonium chloridemixing (exchange) was repeated 8 times to fully convert the material tothe ammonium exchanged RHO(3.2). After filtering, the material wasrinsed 3 times with a 3-fold excess (weight % basis) of DI water and wasdried overnight at 90° C. Typically, 75 g of the ammonium exchangedRHO(3.2) was placed in a ceramic dish and calcined in a purged FisherScientific muffle furnace. While the oven was purged with ambient air at5 L/min, the material was heated at a rate of 0.8° C./min in to 550° C.and calcined at this temperature for 24 hrs to prepare the protonexchanged RHO composition. Solid State ²⁹Si NMR demonstrated that somede-alumination had occurred during calcination, leading to a Si/Al of3.5. Calcination conditions and products from ammonium substituted RHOmaterials are shown in Table 2.

TABLE 2 Calcination of NH₄-substituted RHO materials CalcinationCalcination time Temperature Product Starting RHO (hrs) ° C. Composition(NH₄)_(11.4)RHO(3.2) 24 550 H_(10.7)RHO(3.5) (NH₄)_(9.8)RHO(3.9) 24 550H_(9.2)RHO(4.2) Li_(7.8)(NH₄)₂RHO(3.9) 24 550 Li_(7.8)H₂RHO(3.9)Li_(6.8)(NH₄)₃RHO(3.9) 24 550 Li_(6.8)H₃RHO(3.9) Li_(5.8)(NH₄)₄RHO(3.9)24 550 Li_(5.8)H₄RHO(3.9)

Example 6: Synthesis of H_(9.2)RHO(4.2)

RHO(3.9), Na_(6.8)Cs_(3.0)RHO(3.9), from Example 2 was mixed with a10-fold excess (mole % basis) of 1M ammonium chloride solution at 90° C.for at least 4 hrs. After mixing, the material was filtered. Theammonium chloride mixing (exchange) was repeated 8 times to fullyconvert the material to the ammonium exchanged RHO(3.9). Afterfiltering, the material was rinsed 3 times with a 3-fold excess (wt. %basis) of DI water and was dried overnight at 90° C. Typically, 75 g ofthe ammonium exchanged RHO(3.9) was placed in a ceramic dish andcalcined in a purged Fisher Scientific muffle furnace. While the ovenwas purged with ambient air at 5 L/min, the material was heated at arate of 0.8° C./min in air to 550° C. and calcined at this temperaturefor 24 hrs to prepare the proton exchanged RHO composition, as shown inTable 2. Solid State ²⁹Si NMR demonstrated that some de-alumination hadoccurred during calcination, leading to a Si/Al of 4.2.

Example 7: Synthesis of Mixed Li,HRHO(3.9) and Li,HRHO(3.2) Zeolites

Li_(9.0)Na_(0.8)RHO(3.9), from Example 4, was mixed with 2:1, 3:1, and4:1 stoichiometric ratios of 1M ammonium chloride solution at 90° C. forat least 4 hrs. After mixing, the material was filtered. Afterfiltering, the material was rinsed 3 times with a 3-fold excess (wt. %basis) of DI water and was dried overnight at 90° C. Typically, 5-10 gof the partially ammonium exchanged RHO(3.9) samples were placed in aceramic dish and calcined in a purged Fisher Scientific muffle furnace.While the oven was purged with ambient air at 5 L/min, the material washeated at a rate of 0.8° C./min in air to 550° C. and calcined at thistemperature for 24 hrs to prepare the mixed Li₇.8H₂RHO(3.9),Li_(6.8)H₃RHO(3.9), and Li_(5.8)H₄RHO(3.9) compositions, as shown inTable 2. The extent of ion exchange was confirmed by inductively coupledplasma optical emission spectroscopy (ICP-OES). An analogous process wascarried out to prepare mixed Li,HRHO(3.2) zeolites, includingH₆Li_(5.4)RHO(3.2), after exchanging Li_(11.4)RHO(3.2) from example 4with a 6:1 stoichiometric ratio of 1M ammonium chloride solution at 90°C. for 4 hrs.

While the resulting LiHRHO(3.2) composition is referred to in thisapplication as H₆Li_(5.4)RHO(3.2), based on the starting partiallyammonium exchanged RHO(3.2); it is likely that some de-alumination to aslightly higher framework Si/Al ratio has occurred. ICP-OES resultssuggest the actual Si/Al may be closer to 3.5.

Example 8: Adsorption Rate Uptake Measurements

The mass transfer properties of the adsorbents were evaluated using astandard volumetric adsorption apparatus. The experiment consisted ofexposing an adsorbent sample, which is initially at vacuum and 30° C.,to a measured amount of O₂, N₂, or Ar at 760 Torr (101 kPa). The changein pressure was then followed as a function of time. The pressure timedata is then subtracted from a similar pressure history using the sameweight of quartz beads in the place of the adsorbent sample to obtain aplot of the amount of gas adsorbed as a function of time, also known asan uptake curve. From the initial slope of the uptake curve, a diffusionparameter for the test gas in units of inverse time (sec⁻¹) can beobtained. It is understood that the heat dissipation from adsorbent dueto this step change in adsorbate loading during the kinetic measurementcan affect the diffusion parameter when isothermal model is used in thecalculation of the parameters. It is important to note that the heat ofadsorptions for oxygen, nitrogen and argon on the RHO adsorbentsconsidered here are significantly lower than most of the known zeoliteadsorbents. Therefore, the diffusional parameter calculated under theassumption of isothermal behavior should be a reasonable estimate of thediffusion parameter.

A pseudo-equilibrium capacity can be defined for a given adsorbentsample over the timeframe of the experiment as follows. The pressurereduction of a gas over an adsorbent sample weighing 2.2 g is measuredstarting at 760 Torr (101 kPa) until the rate of pressure reduction is<1 Torr/min. A term Pmax is defined as the total pressure reduction orgas uptake over a 2.2 g sample of adsorbent after subtraction from thepressure reduction of the 2.2 g glass bead blank. The Pmax together withthe system volume, thus defines a pseudo-equilibrium capacity(*). ThesePmax values are given for various adsorbents in Table 3.

TABLE 3 Structural and Adsorption Data for Adsorbents of the Inventionand Prior Art Materials. Required minimum 8-ring occupancy RHO of non-Particle Kinetic Kinetic O₂ N₂ Ar Unit Cell proton O₂ size SelectivitySelectivity Pmax Pmax Pmax O₂ Pmax*/ O₂ Pmax*/ Adsorbent axis (Å)cations D/r² (μm) O₂/N₂ O₂/Ar (torr) (torr) (torr) N₂ Pmax* Ar Pmax*Li_(9.5)Na_(1.6)Cs_(0.3)RHO(3.2)^(d) 14.219 3.4 9.84E−05 2.5 ND 100 36ND ND >10 >10 Li_(11.4)RHO(3.2) 14.167 3.4 1.00E−04 2.5 ND ND ND ND NDND ND H₆Li_(5.4)RHO(3.2)^(c) ND 0 1.01E−01 2.5 3.70E+01 >50 50 75 420.67 1.2 Li_(9.4)Ca_(1.0)RHO(3.2) 14.155 2.4 ND 2.5 ND ND ND ND ND ND NDLi_(9.0)Zn_(1.2)RHO(3.2)^(c) 14.219 2.2 1.97E−03 2.5 17.2 17.8 43 37 51.15 8.2 Li_(9.0)K_(0.8)RHO(3.9)^(c) 14.255 1.8 6.70E−03 1.0 37 41 47 5314 0.89 3.3 Li_(9.0)Na_(0.8)RHO(3.9)^(c) ND 1.8 6.70E−03 1.0 30 100 5365 5 0.82 11.6 Li_(8.3)Cs_(1.5)RHO(3.9) 14.362 1.8 5.49E−02 1.0 39 30146 81 38 0.57 1.2 Li_(8.0)Zn_(1.7)RHO(3.2)^(c) ND 1.7 3.14E−03 1.0 22.423 43 43 3 1 12.3 Zn_(4.1)Na_(1.6)RHO(3.9) ND 1.6 3.10E−01 1.0 13.5 3556 160 56 0.35 1.0 Li_(9.2)H_(0.6)RHO(3.9) ND 1.2 1.77E−02 1.0 65 117 4679 14 0.58 3.2 Li_(9.2)RHO(4.2) ND 1.2 2.40E−01 1.0 51 323 52 0.11 0.180.08 1.4 Li_(7.8)Ca_(1.0)RHO(3.9) 14.211 0.8 ND 2.5 ND ND ND ND ND ND NDLi_(7.4)Zn_(1.2)RHO(3.9) 14.300 0.6 3.52E−02 1.0 40.3 207 48 80 21 0.62.3 Li_(6.0)H_(1.8)Zn_(1.0)Na_(0.6)RHO(3.6) ND 0.6 2.04E−01 1.0 58 48551 0.11 0.21 0.10 1.1 Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) 14.330 0.53.70E−01 1.0 53 525 52 118 47 0.44 1.1 Zn_(4.1)Li_(1.1)Na_(0.5)RHO(3.9)14.489 0.5 9.73E−01 1.0 23 45 52 144 52 0.36 1.0Cu_(3.4)Li_(2.8)Na_(0.2)RHO(3.9) 14.867 0.2 7.60E−01 1.0 2.1 0.83 60 9262 0.65 1.0 Li_(7.8)H_(2.0)RHO(3.9) ND 0 3.60E−02 1.0 51 171 53 85 290.62 1.8 Zn_(5.7)RHO(3.2) 14.437 0 9.00E−02 2.5 43 282 39 91 30 0.43 1.3Li_(6.8)H_(3.0)RHO(3.9) ND 0 1.10E−01 1.0 43 224 51 81 39 0.63 1.3Zn_(4.9)RHO(3.9) 14.541 0 3.70E−01 1.0 10 45 50 141 48 0.35 1.0Li_(5.8)H_(4.0)RHO(3.9) ND 0 3.73E−01 1.0 31 120 52 80 47 0.65 1.1H_(10.7)RHO(3.5) ND 0 5.80E−01 1.0 4.1 3.7 39 50 39 0.78 1.0Cu_(5.7)RHO(3.2) 14.915 0 6.10E−01 2.5 1.4 0.7 52 72 53 0.72 1.0H_(9.2)RHO(4.2) 15.035 0 7.10E−01 1.0 1.3 0.8 69 91 76 0.76 0.9Cu_(4.9)RHO(3.9) 14.938 0 7.20E−01 1.0 1.6 0.94 66 92 68 0.72 1.0 4A NANA 4.80E−01 5 25 16 41.8 113 39 0.37 1.1 CMS NA NA 8.83E−03 4.5 36.564.7 107.3 107 107 1.0 1.0 RS10^(a) NA NA 9.94E−03 ND 35.0 35 ND ND NDND ND Ba-RPZ-3^(b) NA NA 2.43E−03 ND 1.0 6 ND ND ND ND ND ^(a)S. Farooq,Gas Separations and Purification, Vol. 9, No. 3, pp 205-212. ^(b)S.Kuznicki, B. Dunn, E Eyring and D. Hunter, Separation Science andTechnology, 2009, 44: 7, pp 1604-1620. ^(c)N₂ and Ar not fullyequilibrated and their rates are overestimated. The Pmax reportedreflects pressure drop over 30 minutes of measurement. ^(d)O₂ not fullyequilibrated and its rate is overestimated. The Pmax reported reflectspressure drop over 30 minutes of measurement. NA = Not applicable ND =Not Determined

The theory behind the derivation of the diffusion parameter is given bythe Fickian Diffusion model in which a special case of a more rigorouschemical potential driving force model for adsorbate transport withinthe adsorbent particle is used. The effect of finite system volume onoverall kinetics is also considered. The analytical form of the model isgiven by Equation 2:

${f(t)} = {1 - {6{\sum\limits_{n = 1}^{\infty}\frac{\exp \left( {- \frac{D\mspace{14mu} P_{n}^{2}T}{r^{2}}} \right)}{\frac{9\; \gamma}{1 - \gamma} + {\left( {1 - \gamma} \right)P_{n}^{2}}}}}}$

Where f(t) is the fractional uptake, D is the intra-crystallinediffusivity, r is crystal radius (diffusional length scale), t is thetime, γ is the fraction of adsorbate ultimately adsorbed by theadsorbent, and P_(n) are the non-zero roots of Equation 3:

${\tan \; P_{n}} = \frac{3\mspace{14mu} P_{n}}{3 + {\left( {\frac{1}{\gamma} - 1} \right)P_{n}^{2}}}$

as set forth in chapter 6 of Ruthven, D. M. Principles of Adsorption andAdsorption Processes, John Wiley and Sons, New York, 1984.

Kinetic selectivity parameters were measured for the RHO compositions ofthis invention and compared with other zeolite and carbon molecularsieve (CMS) materials tested internally and from the literature. All ofthe RHO samples described herein were activated under vacuum (<10 mPa)at 400° C. for at least 8 hours to remove water and CO₂ prior toadsorption measurements. The results are compiled in Table 3.

The ambient temperature data in Table 3 and FIGS. 6 and 7 represent awide range of RHO phases with Si/Al between 3.2 and 4.2, which showsignificantly better kinetic selectivity for O₂ vs. both Ar and N₂ thanCMS or known commercial zeolites, while maintaining O₂ uptake rates thatare 10 to 50× faster than CMS. Surprisingly, the rates are some 1000 to5000× faster than the Li-rich RHO materials reported by Corbin in U.S.Pat. No. 7,169,212.

CMS, RS10, 4A, and the MOF Ba-RPZ-3 show poorer selectivities at slowerrates, apart from 4A zeolite. While 4A zeolite shows very fast O₂uptake, its selectivity for O₂ vs. Ar adsorption is much lower than mostof the RHO materials.

The O₂ vs. N₂ equilibrium selectivity of low-silica zeolites istypically less than 0.5, owing to the stronger quadrupole interactionsbetween N₂ and electropositive extra framework cations of the zeolite.The equilibrium data in Table 3 demonstrate that a number of the RHOzeolites also show this behavior, but as can be seen in FIG. 5, the O₂vs. N₂ equilibrium selectivity can be significantly improved byincreasing the number of protons exchanged for Li⁺ in aLi_(9.0)Na_(0.8)RHO(3.9) zeolite starting zeolite. Complete exchange tothe fully protonated RHO results in slight de-alumination, but alsoincreases the O₂/N₂ equilibrium selectivity to 0.76. Unfortunately, thelack of 8-ring distortion by these cations leads to negligible kineticselectivity between O₂ and N₂. By balancing improved O₂ vs. N₂equilibrium selectivity with high kinetic selectivity and rate, itappears that an optimum is achieved on exchange of 3-4 protons (FIGS. 1and 5).

Example 9: Isotherm Measurements

Isotherms at various temperatures were measured onLi_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO (3.9), Li_(6.8)H_(3.0)RHO(3.9), theLi_(9.5)Na_(1.6)Cs_(0.3)RHO(3.2) analog to the material of Corbin, andZn_(4.1)Na_(1.6)RHO(3.9) using a 3FLEX Surface Characterization Unitfrom Micromeritics for pressures measured up to 1 atm absolute or on aVTI HPA 300 Adsorption Unit for pressures measured up to 10 atmabsolute. Isotherms were collected for O₂, N₂, and Ar at 5, 23, and 45°C. for the first three samples and on N₂ and Ar at 23 and 45° C. forZn_(4.1)Na_(1.6)RHO(3.9). Isotherm plots comparing O₂ and Ar capacitiesmeasured up to 10 atm are shown in FIG. 4a forLi_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO (3.9).

The isotherms measured up to 1 atm forLi_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) are shown in FIG. 4b , and an O₂vs. Ar equilibrium selectivity of close to 1 is observed at 23° C.Effective equilibrium selectivity for O₂ vs. Ar is observed as theadsorption temperature is dropped below 23° C., which is likelyassociated with temperature dependent contraction and reduced vibrationof the RHO 8 rings. The O₂ and N₂ isotherms for Li_(6.8)H_(3.0)RHO(3.9)in FIG. 8 and Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) in FIG. 9 showimproved O₂/N₂ equilibrium selectivity for Li_(6.8)H_(3.0)RHO(3.9) vsLi_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9). The isotherms for theLi_(9.5)Na_(1.6)Cs_(0.3)RHO(3.2) are shown in FIG. 10. A curious featureof the isotherm data for this composition is that the O₂ capacities at 5and 23° C. are nearly identical. It is unclear if this is a consequenceof the extreme slowness of the O₂ uptake at 5° C., or if the slightcontraction of the 8 ring windows going from 23 to 5° C. leads to feweraccessible sites for the oxygen. The N₂ and Ar isotherms forZn_(4.1)Na_(1.6)RHO(3.9) are shown in FIG. 11.

Example 10: Argon Production Via Ambient Temperature PSA withLi_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9)

A 2-bed multi-step pressure swing adsorption (PSA) process cycle is usedto evaluate process performance indicators in the form of primaryproduct (Ar) recovery and productivity usingLi_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) adsorbent. The adsorbentcharacteristics and adsorbent bed characteristics used in the simulationare shown in Table 4.

TABLE 4 Characteristics of the bed and adsorbent as well as theoperating conditions used to evaluate process performance indicators.Adsorbent Characteristics Adsorbent typeLi_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) Li_(6.8)H_(3.0)RHO(3.9)Zn_(4.1)Li_(1.1) Na_(0.5)RHO(3.9) Adsorbent diameter (m) 0.002 0.0020.002 Total void fraction 0.65 0.65 0.65 Interstitial void fraction 0.400.40 0.40 Bulk density (kg/m³) 800.92 800.92 800.92 Rate constant (D/r²)for Oxygen (1/s) 0.3700 0.1100 0.9730 Nitrogen (1/s) 0.00698 0.002560.0423 Argon (1/s) 0.00070 0.00049 0.0216 Bed Characteristics Number ofbeds 2 2 2 Bed length (m) 2.40 2.40 2.40 Bed inside diameter (m) 2.0 2.02.0 Middle port from bottom 1.35 1.35 1.35 (m) Bed wall thickness (m)0.016 0.016 0.016 Feed end head space 0.096 0.096 0.096 (m³) Exit endhead space (m³) 0.113 0.113 0.113 Operating Conditions Temperature (°C.) 37.78 37.78 37.78 ¹Feed pressure (bara) 7.90 7.90 7.90 ²Purgepressure (bara) 1.09 1.09 1.08 ³Purge to feed ratio 0.14 0.14 0.36 Feedmole fraction for Oxygen 0.20 0.20 0.20 Nitrogen 0.0005 0.0005 0.001Argon 0.7995 0.7995 0.799 Cycle time (s) 60 64 300 Process PerformancePrimary impurity O₂ O₂ N₂ removed Primary product impurity 2.0 2.0 1.0(ppm) Argon recovery (%) 61.44 56.01 38.67 Productivity (nm³/h/m³)283.12 215.51 87.35 ¹Pressure is at the middle of the bed and at the endof feed step, ²Pressure is at the middle of the bed and at the end ofpurge step, ³Ratio is based on average purge and average feed flows inlb-moles/h.

The PSA cycle is operated by following the sequence shown in FIG. 12 ata feed pressure of 7.90 bara and a temperature of 37.78° C. At the startof the cycle (F1/RP1), the bed is pressurized to the highest-pressurelevel of the cycle with the addition of primary product and feed gasfrom top and bottom end of the bed, respectively. No product iswithdrawn during this phase of process cycle. The feed step (F2 and F3)is continued until the mass transfer zone of preferentially adsorbedcomponent (O₂) reaches the exit end of the bed without substantiallybreaking through it, and the un-adsorbed gas (first purified Ar) isdischarged from the outlet end of bed. The feed gas consists of 79.95mole % Ar, 20.00 mole % O₂, and balance N₂. After the feed step, the bedpressure is reduced to 7.79 bara through stopping feed flow andextracting product from top end of the bed (CoD). At the termination ofCoD step, there is an idle step (I1). Then, the bed (EQD1) is connectedwith the second bed undergoing equalization re-pressurization step(EQR1) and a portion of the void as well as desorbed gas is transferredfrom the product end of first bed to the product end of second bed, thuslowering the first bed pressure to approximately 6.67 bara at the end ofthis step (EQD1). Following this step, a dual end equalizationde-pressurization step (DEQD2) is introduced to transfer moreco-adsorbed as well as void gases from the first bed to the second bedfrom top as well as middle of the first bed to the bottom of the secondbed until the pressure of the first bed goes down to approximately 4.54bara. The dual end depressurization step is then coupled withcounter-current depressurization step (DEQD3/CnD1) which further reducesthe pressure of the bed to approximately 3.03 bara. The column is thencounter-currently de-pressurized (CnD2) and purged (PU1 and PU2) at 1.09bara, and thereafter, re-pressurized (RP2 and RP3) counter-currentlywith primary product gas at 1.51 bara. Following the re-pressurizationstep, the column is subsequently pressurized through pressureequalization re-pressurization steps (EQR1, DEQR2, and DEQR3) to bringback the pressure level for initiation and repetition of the cycle. Notethat three idle steps (I1, I2, and I3) are incorporated into the cycleschedule and during this step the bed is isolated and all valves leadingto it are closed.

With all the steps, the full cycle completes in 60 seconds. The netO₂-free (2 ppm O₂ in primary product) Ar recovery from the feed gas is61.44% and the productivity is 283.12 Nm³/h/m³ bed. The Ar recovery andproductivity benefits of RHO type adsorbent are compared with carbonmolecular sieve (CMS) based PSA process in FIG. 13 for 1, 2, and 5 ppmO₂ in primary product. Note that the process conditions and the cyclesequence are kept same as mentioned above for the comparison. Theseperformance benefits result in the following commercial advantages:

Argon recovery is improved from about 40 to 45% in CMS to about 61% inRHO adsorbents for product O₂ purity range from 1 to 5 ppm. In the caseof process integration, this higher recovery reduces recycle back to thedistillation column.

Argon productivity is improved about 5 times versus a CMS based PSAprocess, reducing bed size and cost and enabling crude argonpurification of larger feed flows.

Use of RHO zeolites in place of CMS eliminates the safety concern ofcombustible carbon particles in an O₂-rich environment if the PSA wastestream is recycled back to the distillation column is used. Thus,filters present in CMS Argon PSA are eliminated, resulting in reducedequipment and capital expenditure.

Example 11: Argon Production Via Ambient Temperature PSA withLi_(6.8)H_(3.0)RHO(3.9)

This example compares the process performances in terms of argonrecovery and productivity of the RHO composition Li_(6.8)H_(3.0)RHO(3.9)with the RHO composition Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9). The2-bed PSA process cycle described above is used to evaluate the processperformances. The adsorbent and bed characteristics as well as operatingconditions are summarized in Table 4. For both cases, the PSA process isindependently optimized for final evaluation. It is worth noting thatlike Example 10, this example deals with primarily oxygen removal by thePSA process. The process performances are summarized in Table 4.Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) is capable of enhancing recoveryand productivity by about 10% and about 31% respectively due to itshigher oxygen rate and higher oxygen over argon selectivity. It is worthnoting that the simulation results are obtained for the purpose ofdemonstration. A different combination of argon recovery andproductivity can be obtained by changing different parameters. Forexample, a productivity gain can be achieved by reducing cycle timewhich would affect the argon recovery to some extent.

Example 12: Simulation of Removal of Trace N₂ from Ar with Zn_(4.1)Li_(1.1) Na_(0.5)RHO(3.9)

This example deals with the removal of predominantly trace nitrogen fromcrude argon stream using a nitrogen selective RHO adsorbent,Zn_(4.1)Li_(1.1) Na_(0.5)RHO(3.9). The 2-bed PSA cycle mentioned withExample 10 is used to evaluate the efficiency of this adsorbent forremoving trance nitrogen from argon stream. The adsorbent and bedcharacteristics as well as the operating conditions are summarized inTable 4. It is worth mentioning that the adsorbent is also kineticallyselective towards nitrogen, therefore, effective selectivity (which is afunction of equilibrium and kinetic selectivity) is essentially higher.Another important characteristic of the adsorbent is that the isothermsof nitrogen, oxygen, and argon are less steep than commonly knownthermodynamically selective zeolites. This means that a smaller purge tofeed ratio is sufficient to maintain a reasonable performance.

The results from the simulation is summarized in Table 4. With 1000 ppmnitrogen in the feed and 1.0 ppm nitrogen in the product stream, anargon recovery and productivity of 38.67% and 87.35 nm³/h/m³ can beachieved at 37.78° C. and 7.90 bara.

Example 13: Simulation of Layered Bed for Ar PSA with Trace N₂ and O₂Removal from Ar Stream Using Adsorbents from Present Invention in BothLayers

This example is presented to illustrate the performance of a layered bedcomprising of an equilibrium selective layer and a kinetically selectivelayer to simultaneously remove oxygen and nitrogen to produce very highpurity argon stream using a pressure swing adsorption process at ambienttemperature. The novelty can be understood through arrangement of thetwo layers in the same column in a pressure swing adsorption process atambient temperature.

Two cases (Case 1 and Case 2) are created to better understand theinvention. In Case 1, the adsorbent materials inside the columns arearranged in two separate layers: first layer of adsorbent with kineticselectivity to one of the contaminant gases (in this case, oxygen),preferentially where the product gas (in this case argon) has very slowdiffusion kinetics. The second layer is comprised of an adsorbentmaterial where the separation is enabled by differences in equilibriumcapacities, where the contaminant gas (mainly nitrogen) is more adsorbedthan the product gas. The Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) isused in the first layer and the Zn_(4.1)Li_(1.1) Na_(0.5)RHO(3.9) isused in the second layer. In Case 2, a reverse scenario is created,i.e., equilibrium adsorbent as the first layer and kinetic adsorbent inthe second layer. The adsorbent and bed characteristics, layeringinformation, and operating conditions can be found in Tables 4 and 5.Note that, the total bed length is maintained constant for both cases.

TABLE 5 Process summary for simultaneous oxygen and nitrogen removalusing a layered bed conventional 2-bed PSA process. Case 1 Case 2 BedCharacteristics Layering configuration Feed end O₂ removal (kinetic)layer N₂ removal (equilibrium) Product end N₂ removal (equilibrium) O₂removal (kinetic) layer Adsorbent for O₂ removalLi_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9)Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) Adsorbent for N₂ removalZn_(4.1)Li_(1.1)Na_(0.5)RHO(3.9) Zn_(4.1)Li_(1.1)Na_(0.5)RHO(3.9) Numberof beds 2 2 Total bed length (m) 3.9 3.9 O₂ removal layer length 1.5 2.4Bed inside diameter (m) 2.0 2.0 Middle port from bottom 0.90 1.5Operating Conditions Temperature (° C.) 37.78 37.78 ¹Feed pressure(bara) 7.90 7.90 ²Purge pressure (bara) 1.07 1.09 ³Purge to feed ratio0.12 0.15 Feed mole fraction for Oxygen 0.20 0.20 Nitrogen 0.001 0.001Argon 0.799 0.799 Cycle time (s) 170 120 Process Performance O₂ inprimary product 2.0 2.0 N₂ in primary product 1.0 1.0 Argon recovery (%)28.63 33.32 Productivity (nm³/h/m³) 42.88 61.42 ¹Pressure is at themiddle of the bed and at the end of feed step, ²Pressure is at themiddle of the bed and at the end of purge step, ³Ratio is based onaverage purge and average feed flows in lb-moles/h.

The aforementioned processes (Cases 1 and 2) are applied to produce aproduct argon stream containing nitrogen and oxygen of 1.0 ppm and 2.0ppm, respectively at 37.78° C. and 7.90 bara. As can be seen from Table5, the feed gas consists of 0.1 mole % nitrogen, 20 mole % oxygen andbalance argon. The 2-bed PSA cycle mentioned with previous examples isused for performance evaluation. From Table 5, it is clear that thesecond case performs better than Case 1 (about 16% better argon recoveryand about 43% better productivity).

The purge is more efficient when an equilibrium layer is added in thefeed end followed by a kinetic layer in the product end. The RHOadsorbents mentioned here are both capable of removing both oxygen andnitrogen to some extent from the argon stream. Thus, the nitrogen masstransfer zone from equilibrium layer can be pushed further towards thekinetic layer. This implies that the bed utilization is higher inconjunction with effective purge.

An interesting feature with the layering configuration is that themiddle bed pressure transfer used in the PSA cycle plays an importantrole in providing an incremental benefit when equilibrium layer isplaced in the feed end. The optimum location for the middle bed port isat the interface between equilibrium layer and kinetic layer as can beseen from Table 5. No significant benefit is observed with middle bedport location when kinetic layer (O₂ removal layer) is placed in thefeed end.

With the preferred layering sequence discussed in Table 5, Case 2, it isobserved from examples in FIG. 14 that increasing the proportion of O₂selective kinetic layer can enhance argon recovery at the expense ofreduced productivity. The PSA cycle configuration, adsorbent and bedcharacteristics, operating conditions, and product O₂ and N₂ levels arekept same as Case 2 mentioned above. The total bed length is also keptconstant at 3.9 m.

TABLE 6 Process summary for simultaneous oxygen and nitrogen removalusing a preferred layering configuration as defined in Table 5, Case 2utilizing known adsorbents. Layering configuration Feed end layer N₂removal N₂ removal N₂ removal N₂ removal (equilibrium) layer Product endlayer O₂ removal O₂ removal O₂ removal O₂ removal (kinetic) Adsorbentfor O₂ Li_(6.8)H_(3.0)RHO(3.9) Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9)Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9)Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) removal layer Adsorbent for N₂Zn_(4.1)Li_(1.1)Na_(0.5)RHO AgLiLSX CaX NaX removal layer Number of beds2 2 2 2 Total bed length (m) 3.9 3.9 3.9 3.9 O₂ removal layer length 2.42.4 2.4 2.4 Bed inside diameter (m) 2.0 2.0 2.0 2.0 Middle port frombottom 1.5 1.5 1.5 1.5 For N₂ layer ¹K_(H, N2) (mmole/gm/bara) 0.71 2.383.69 0.37 Effective N₂/Ar 6.55 18.77 17.33 3.32 Temperature (° C.) 37.7837.78 37.78 37.78 ²Feed pressure (bara) 7.90 7.90 7.90 7.90 ³Purgepressure (bara) 1.07 1.10 1.12 1.08 ⁴Purge to feed ratio 0.11 0.21 0.210.12 Feed mole fraction for Oxygen 0.20 0.20 0.20 0.20 Nitrogen 0.0010.001 0.001 0.001 Argon 0.799 0.799 0.799 0.799 Cycle time (s) 120 100120 260 O₂ in primary product 2.0 2.0 2.0 2.0 N₂ in primary product 1.01.0 1.0 1.23 Argon recovery (%) 32.18 33.46 9.90 6.72 Productivity(nm³/h/m³) 58.03 78.05 15.89 4.79 ¹K_(H, N2) is the Henry's law constantfor N₂ at 37.78° C. ²Pressure is at the middle of the bed and at the endof feed step, ³Pressure is at the middle of the bed and at the end ofpurge step, ⁴Ratio is based on average purge and average feed flows inlb-moles/h.

Example 14: Simulation of a Layered Bed for Ar PSA with Trace N₂ and O₂Removal from Ar Stream Using Li_(6.8)H_(3.0) RHO(3.9) as O₂ RemovalLayer and Zn_(4.1) Li_(1.1) Na_(0.5)RHO(3.9) as N₂ Removal Layer

This example illustrates the performance of a layered bed pressure swingadsorption (PSA) process for trace N₂ removal and for O₂ removal, inwhich the Zn_(4.1)Li_(1.1)Na_(0.5)RHO(3.9) adsorbent is used as N₂removal layer and the Li_(6.8)H_(3.0)RHO(3.9) adsorbent is used as O₂removal layer. The adsorbents inside the PSA bed are arranged in thesame manner as explained with Case 2 in Example 13: N₂ removal layer isused in the feed end (as first layer) and O₂ removal layer is in theproduct end (as second layer).

An adsorption process simulator is used to evaluate the layered bed PSAperformance using the above layering configuration for a feed gascomposition of 0.1 mole % nitrogen, 20.0 mole % oxygen and balanceargon. The feed pressure is 7.90 bara, purge pressure is 1.07 bara andtemperature is 37.78° C. Note that the 38.46% of the total bed length isfilled with adsorbent for N₂ removal layer and 61.54% of the length isfilled with O₂ removal layer. The adsorbent and bed characteristics,layering information, and operating conditions can be found in Table 6.The 2-bed PSA cycle mentioned with previous examples is used forperformance evaluation.

The process is simulated to produce a product argon stream containingnitrogen and oxygen of 1.0 ppm and 2.0 ppm, respectively. The processperformance is summarized in Table 6. It is clear from the table thatthe adsorbent Li_(6.8)H_(3.0)RHO(3.9) is not as efficient as theadsorbent Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9). WithLi_(6.8)H_(3.0)RHO(3.9) the argon recovery is 3.4% worse and theproductivity is 5.5% worse than withLi_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9).

Example 15: Simulation of a Layered Bed for Ar PSA with Trace N₂ and O₂Removal from Ar Stream Using the Inventive RHO,Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5) RHO (3.9) as O₂ Removal Layer and KnownAdsorbents for N₂ Removal Layer

This example is presented to illustrate the performances of layered bedcomprising of known adsorbents for trace nitrogen removal (selected fromthe group formed by mordenite, ferrierite, clinoptilolite and the typeA, X, Y, or mixture therefrom) and the inventive RHO,Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9) for oxygen removal layer tosimultaneously remove oxygen and nitrogen to produce very high purityargon stream using a pressure swing adsorption (PSA) process at ambienttemperature. The adsorbents inside the PSA bed are arranged in the samemanner as explained with Case 2 in Example 13: nitrogen removal layer isin the feed end (as first layer) and oxygen removal layer is in theproduct end (as second layer).

The inventors have identified that employing a suitable conventionaladsorbent for nitrogen removal layer (selected from the group formed bymordenite, ferrierite, clinoptilolite, chabazite and the type A, X, Y,or mixture therefrom) can enhance the overall performance of the layeredbed process. The criterion used for selecting a suitable adsorbent fornitrogen removal is based on the Henry's Law constant for nitrogen(K_(H,N2)). The Henry's Law constant for an adsorption isotherm isdefined as the initial isotherm slope. See, for example, “PhysicalAdsorption of Gases”, Young, D. M. and Crowell, A. D., p. 104(Butterworths, London 1962). The unit of the constant is in amount ofgas adsorbed per unit weight of adsorbent per unit of pressure (e.g.,mmole of gas adsorbed/gm of adsorbent/bar absolute pressure).

An adsorption process simulator is used to evaluate the layer bed PSAperformance using the above layering configuration with differentadsorbents for nitrogen removal for a feed gas composition of 0.1 mole %nitrogen, 20.0 mole % oxygen and balance argon. The feed pressure is7.90 bara and temperature is 37.78° C. Note that the 38.46% of the totalbed length is filled with adsorbent for nitrogen removal layer and61.54% of the length is filled with oxygen removal layer. The adsorbentand bed characteristics, layering information, operating conditions andprocess performances are summarized in Table 6. The 2-bed PSA cyclementioned with previous examples is used for performance evaluation.

It is clear from Table 5, Case 2 and Table 6 that there is a preferredrange of 0.5 to 3.0 mmole/gm/bara at 37.78° C. for Henry's law constantfor nitrogen which significantly improves argon recovery andproductivity under layering configuration. For example, when the productend of the bed (second layer) consists essentially of the RHO adsorbentfrom the invention (predominantly for oxygen removal) and the feed end(first layer) consists of AgLiLSX (predominantly for nitrogen removal),the performances are significantly better than other adsorbentsconsidered in Table 6. Note that the K_(H,N2) for AgLiLSX is 2.38mmole/gm/bara and the effective selectivity for nitrogen over argon(function of Henry's law constant and kinetics as set forth in “PressureSwing Adsorption”, Ruthven, D. M.; Farooq, S. and Knaebel, K. S., p. 52(VCH, New York, 1994) is 17.33 at 37.78° C. Above the suitable range ofthe Henry's constant for nitrogen, the adsorbate (nitrogen) is morestrongly adsorbed and it is impractical to use a PSA process for gasremoval. For example, the CaX adsorbent, which has a K_(H,N2) of 3.69mmole/gm/bara at 37.78° C., shows a significant performance drop, eventhough it has an effective nitrogen over argon selectivity similar toAgLiLSX, as can be seen in Table 6. Alternatively, too low of Henry'sconstant for nitrogen having poor effective nitrogen over argonselectivity would result in very poor performances as can be seen fromTable 6 with NaX adsorbent. Note that the performances from theinventive Zn_(4.1)Li_(1.1) Na_(0.5)RHO(3.9) for nitrogen removal havingreasonable Henry's constant performs quite well under layering model forsimultaneous oxygen and nitrogen removal (Table 5, Case 2) even thoughits effective nitrogen over argon selectivity is relatively low at 6.55.

1. A RHO zeolite having a Si/Al ratio of from 3.2 to 4.5 and containingnon-proton extra-framework cations, wherein the zeolite contains at most1 proton per unit cell, and wherein the size, number and charge of theextra-framework cations that are present in the zeolite are such that 1or fewer non-proton extra-framework cations per unit cell are requiredto occupy 8-ring sites.
 2. The zeolite of claim 1, wherein the zeolitehas a Si/Al ratio of from 3.6 to 4.2.
 3. The zeolite of claim 1, whereinthe non-proton extra-framework cations comprise Li⁺, Mg²⁺, Mn²⁺, Fe²⁺,Co²⁺, Ni²⁺, Cu²⁺ and/or Zn²⁺ cations.
 4. The zeolite of claim 1, whereinthe non-proton extra-framework cations comprise Li and/or Zn²⁺ cations.5. The zeolite of claim 4, wherein said L⁺ and/or Zn²⁺ cations make upthe majority of the extra-framework cations that are present per unitcell.
 6. The zeolite of claim 4, wherein said L⁺ and/or Zn²⁺ cationsmake up at least 70% of the extra-framework cations that are present perunit cell.
 7. The zeolite of claim 4 wherein said L⁺ and/or Zn²⁺ cationsmake up at least 80% of the extra-framework cations that are present perunit cell.
 8. The zeolite of claim 1, wherein the zeolite has a unitcell axis length of from 14.23 Å to 14.55 Å.
 9. The zeolite of claim 1,wherein the zeolite has a unit cell axis length of from 14.23 Å to 14.50Å.
 10. The zeolite of claim 1, wherein the zeolite has a unit cell axislength of from 14.30 Å to 14.45 Å.
 11. The zeolite of claim 8, whereinthe zeolite is selected from Li_(7.4)Zn_(1.2)RHO(3.9),Li_(5.2)Zn_(1.8)H_(0.5)Na_(0.5)RHO(3.9), Zn_(4.1)Li_(1.1)Na_(0.5)RHO(3.9), Zn_(5.7)RHO(3.2) and Zn_(4.9)RHO(3.9).
 12. The zeoliteof claim 1, wherein the zeolite has a unit cell axis length of from14.45 Å to 14.85 Å.
 13. The zeolite of claim 12, wherein the zeolite isselected from Zn_(4.1)Li_(1.1)Na_(0.5)RHO(3.9) and Zn_(4.9)RHO(3.9). 14.A process of adsorbing oxygen from a feed stream containing oxygen,comprising passing the feed stream through a bed of an adsorbentcomprising the RHO zeolite of claim 8 to adsorb oxygen from the feedstream, thereby producing a product stream depleted in oxygen.
 15. Theprocess of claim 14, wherein the feed stream comprises oxygen and one orboth of nitrogen and argon, and the product stream is depleted in oxygenand enriched in one or both of nitrogen and argon.
 16. The process ofclaim 14, wherein the process is a PSA process comprising an adsorptionstep performed at elevated pressure in which the feed stream is passedthrough a bed of adsorbent comprising the RHO zeolite of claim 8 toadsorb oxygen from the feed stream, and a desorption step performed atreduced pressure in which oxygen from the previous adsorption step isdesorbed from the bed to regenerate the bed for the next adsorptionstep.
 17. A process of adsorbing nitrogen from a feed stream comprisingnitrogen and argon, comprising passing the feed stream through a bed ofan adsorbent comprising the RHO zeolite of claim 12 to adsorb nitrogenfrom the feed stream thereby producing a product stream depleted innitrogen and enriched in argon.
 18. The process of claim 17, wherein theprocess is a PSA process comprising an adsorption step performed atelevated pressure in which the feed stream is passed through a bed ofadsorbent comprising the RHO zeolite of claim 12 to adsorb nitrogen fromthe feed stream, and a desorption step performed at reduced pressure inwhich nitrogen from the previous adsorption step is desorbed from thebed to regenerate the bed for the next adsorption step.
 19. A process ofadsorbing oxygen and nitrogen form a feed stream comprising oxygen,nitrogen, and argon, comprising passing the feed stream through one ormore beds of adsorbent comprising a first adsorbent selective fornitrogen so as to adsorb nitrogen from the feed stream and a secondadsorbent selective for oxygen so as to adsorb oxygen from the feedstream, thereby producing a product stream enriched in argon anddepleted in oxygen and nitrogen, wherein the first adsorbent comprisesthe RHO zeolite of claim
 12. 20. The process of claim 19, wherein thesecond adsorbent comprises the RHO zeolite of claim
 8. 21. The processof claim 19, wherein the first adsorbent has a Henry's law constant fornitrogen of from 0.5 to 3.0 mmole/gm/bara at 37.78° C.
 22. The processof claim 19, wherein the process is a PSA process comprising anadsorption step performed at elevated pressure in which the feed streamis passed through a bed of adsorbent comprising the first and secondadsorbents to adsorb nitrogen and oxygen, respectively, therebyproducing a product stream enriched in argon and depleted in oxygen andnitrogen, and a desorption step performed at reduced pressure in whichoxygen and nitrogen from the previous adsorption step are desorbed fromthe bed to regenerate the bed for the next adsorption step.
 23. Theprocess of claim 22, wherein the bed of adsorbent comprises a firstlayer comprising the first adsorbent and a second layer comprising thesecond adsorbent, the first and second layers being arranged such thatduring the adsorption step the feed stream passes through the firstlayer and contacts the first adsorbent for adsorption of nitrogen beforepassing through the second layer and contacting the second adsorbent foradsorption of oxygen.
 24. A method of preparing a NaCsRHO zeolite havinga desired Si/Al ratio, comprising seeding an RHO preparation mixturewith a NaCsRHO zeolite having the desired Si/Al ratio, and aging andcrystallizing the seeded RHO preparation mixture to form a RHO zeolitehaving a desired Si/Al ratio.
 25. The method of claim 24, wherein theRHO preparation mixture is an aqueous mixture comprising 18-crown-6,water, and sources of silica, alumina, sodium, and cesium.
 26. A methodof preparing a RHO zeolite containing Zn²⁺ and L⁺ cations, comprisingproviding a starting zeolite selected from a NaRHO or (NH₄)RHO orNa(NH₄)RHO zeolite, exchanging some or all of the Na⁺ and/or NH₄ ⁺cations in the starting zeolite with Zn²⁺ cations to provide azinc-exchanged RHO zeolite, and then exchanging some of the cations inthe zinc-exchanged RHO zeolite to provide a RHO zeolite containing Zn²⁺and L⁺ cations.
 27. The method of claim 26, wherein the step ofproviding the starting zeolite comprises exchanging the Na⁺ and Cs⁺cations in a NaCsRHO zeolite with NH₄ ⁺ cations to provide an (NH₄)RHOzeolite, and optionally then exchanging some or all of the NH₄ ⁺ cationsin the (NH₄)RHO zeolite with Na⁺ cations.